Sea Urchins

These prickly animals have highly developed, moveable spines used for defense as well as for moving around. Sea urchins have colonized most types of seabed from rocky shores to deep-sea bottoms.

Sea urchins are echinoderms (spiny-skinned animals). They are related to the starfish, sea lilies, and sea cucumbers. They come in two basic body shapes. Round ones are called regular echinoids. Oval or heart-shaped ones are called irregular echinoids. All urchins have a chalky body called a test, which is covered in spines of varying lengths.
They also have water-filled tubes called tube feet that poke through the test. In regular echinoids the tube feet are arranged in five equal double rows up and down the body. In irregular echinoids the tube feet are arranged to form five petal-like patterns on the animal’s upper surface.

Sea urchins usually live on rocky surfaces and may climb about among large seaweeds and sea grasses. They scrape away at smaller animals and plants living on the surface of the rocks, or chew larger plants with their jaws, which are just inside the mouth. The species known as sand dollars and heart urchins usually burrow in sand and mud.

Water-filled Feet
Like other echinoderms, sea urchins crawl about with the aid of their tube feet.These form part of a complicated network of water-filled canals inside the urchin. The network is known as the water vascular system. The animal uses water pressure to extend the tube feet. They can attach to a surface with their suckers, and they can make stepping movements before