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Marine invertebrates

Starfish and sea urchins

StarfishStarfish Starfish, or sea stars, have sucker-lined arms and mouths on the undersides of their bodies. Most starfish have five arms, but some species have more. Starfish use their arms to prise open shellfish. Once they are open, the starfish pushes its stomach through its mouth into the shell to feed on the soft, fleshy parts inside. If a starfish is attacked, it can lose an arm in order to make its escape. This will then grow back. Starfish belong to a group called echinoderms. The echinoderm group also includes brittle stars, crinoids, sea urchins and sea cucumbers.

A starfish opening mussels with its armsA starfish opening mussels with its arms
Stalked crinoids and brittle star on the seabedStalked crinoids and brittle star on the seabed


The echinoderms live only in marine habitats. They all have tough coverings on their upper sides, made of chalky calcium carbonate plates. The spines on the surface help protect the creatures from predators. Most echinoderms also have the ability to regrow body tissue, including organs and limbs.

Brittle starBrittle star

The underside of a starfish, showing its tube feetThe underside of a starfish, showing its tube feet

Tube feet

Like all echinoderms, starfish and sea urchins have "tube feet". These are small, tube-shaped projections along their arms that allow them to move around. They also use their tube feet for breathing and to help them feed, by passing food to the mouth in the centre of their bodies.

Echinoderms make up the largest group of animals in which no species lives on land or in fresh water.

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