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'Echinoderm' means 'spiny skinned' which is an appropriate name for the members of this group - starfish, sea urchins, brittle stars, and sea cucumbers. Beneath the surface of these animals is an internal skeleton made up from many individual plates which often bear spines and other projections.

Sunstar and BrittlestarAll echinoderms show a five-sided symmetry radiating out from a central disc - most noticeable in the starfish and brittle stars. They have no head or brain, though they do have a nervous system and distinct organ systems. Echinoderms also have a unique way of getting about - on hundreds of little sucker-tipped tube feet. These are hydraulically powered by a water vascular system - a network of canals containing pressurised body fluids. The tube feet extend as fluid flows into them. Tube feet also allow for the uptake of oxygen across the thin membranes and, in starfish, some, at the ends of the arms, are modified into simple eyespots.

Echinoderms are generally bottom dwellers. Starfish are common predators on the rocky shore, preying on molluscs by prising open the shells, extruding their stomach between the shells and secreting enzymes to digest the prey.

Sea urchins tend to be vegetarian, feeding particularly on kelp plants. They have a complex chewing apparatus, known as the Aristotle's lantern, which allows the urchin to graze on tough materials. A sea urchin's spines are effective deterrents to predators but also aid in locomotion.

As their name suggests, brittle stars' arms break very easily, but, like starfish, they are able to regenerate them. Brittle stars are often found in great numbers on the sea bed and may be filter feeders or scavengers.

Sea cucumbers have an elongated body, though this may still be divided into five identical parts. They creep over the sea floor and either eat the sediment, extracting organic material, or they filter feed using modified tube feet. If threatened, sea cucumbers can actually eject some of their guts to deter potential predators, and quickly regenerate their organs.


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