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Echinoderms

Echinoderms are sea animals with a rough outer covering consisting of plates of lime (chalk) embedded in the skin. The name echinoderm means spiny-skinned. There are five kinds of echinoderms: starfish, Brittle-stars, sea-urchins, sea-cucumbers and sea-lilies. Most of them have bodies that show radial symmetry, they are symmetrical about a central point. Echinoderms are found in seas all over the world, except near the Poles. Most echinoderms have separate sexes. They breathe absorbing oxygen from the water through their skins.

Starfish usually have five arms and are often brightly colored. But the beautiful sunstar has twelve arms, and other starfish have up to 50 arms. Along the arms are rows of tube feet, tiny projections with sucker discs. The starfish moves by using its tube feet to grasp objects and then pulling itself along. The center of the starfish contains the animal's mouth and stomach. It feeds by attacking oysters, clams and other molluscs, forcing open their shells with its tube feet and extending its mouth into the molluscs bodies.

Brittle-stars look like starfish, but have long thin arms. Brittle-stars have no suckers, although the arms are used for moving. They feed by capturing food with their arms or by pushing mud containing food particles into their mouths.

Sea-urchins have no arms. The chalky plates are all joined together and the animals are covered with many sharp spines. The mouth is on the underside and is equipped with teeth. The tube feet extend through the plates in the skin and help the animal to move about. When the animals die, the skin and spines drop off leaving a 'skeleton' of plates on the sea bottom. Most sea-urchins are round, but some are shaped like hearts, ovals, or even horse-shoes.

Sea cucumbers are leathery and do look like cucumbers, except that some of the tube feet form tentacles around the mouth. The tentacles are used for feeding. Some species catch small animals while others use their tentacles to scoop mud into their mouths. The animals then digest any food material in it.

Sea-lilies look like plants but are echinoderms animals. They may be attached to the sea-bed by long, chalky tubes resembling stalks. The mouth is surrounded by a 'flower' of feathery tentacles. Some sea-lilies have no stalk and float freely in the sea.