Sea urchins can be found in warm waters, among corals, and also in the oceans, at a depth of up to 7 kilometers. Sea urchins belong to echinoderm animals, at present more than 940 species are known. In their structure hedgehogs consist of a body and needles. The body size varies from 2 to 30 cm, in shape it can be spherical and oblate. Above it is covered with a shell, which prevents the hedgehog from changing the shape of the body. On top of the armor, the needles are movably connected, in different species their size can be from 2 mm to 35 cm. Sharp needles for hedgehogs are absolutely necessary, with their help they move, eat and protect themselves from enemies (some sea urchins have poisonous needles).
In addition, the hedgehogs body has a mouth opening, gills, special organs of balance, and on the lower side of the body there are legs with suckers, which serve to move and dig burrows. And on the upper side of the body, hedgehogs have eyes.
The mouth of the sea urchins is on the lower part of the body. Hedgehogs can chew, they have a so-called Aristotelian canal, equipped with very sharp teeth. With their help hedgehogs scrape off seaweed from stones, chew food, and also move, clinging to the bottom of the teeth.
Some species of hedgehogs are hiding from enemies in cracks, among corals and in burrows, while others prefer to cover themselves with shells, seaweed or bury themselves in the sand.
In food sea urchins can not be called gourmets, as they eat everything: algae, sponges, carrion, small sea stars and crustaceans. Large hedgehogs can eat their small brethren and even mantis crayfish. And those hedgehogs, who live on a sandy bottom and do not bother to search for food at all, swallow large portions of sand and silt, hoping that small living organisms will also get into the body with them.
To the enemies of sea urchins you can include a person. Some species of echinoderms are subject to fishing. In tropical countries, delicacy is the milk and caviar of sea urchins. The shell, which contains a lot of calcium and phosphorus, is used as fertilizer. Pigment of sea urchins contains anti-sediment, which has a general strengthening effect.