Echinoderms have a five fold symmetry. The phylum includes the classes Asteroidea (starfish), Echinoidea (seaurchins and sand dollars), Holothuroidea (sea-cucumbers), Ophiuroidea (brittlestars) and Crinoidea (feather stars). Echinoderms have a calcareous skeleton which is well developed in echinoids as a hard test while in starfish, crinoids and ophiuroids the skeleton is a series of small plates which allow more flexibility. In the holothurians the calcareous skeleton is reduced to small microscopic plates; many sea-cucumbers resemble worms as the five-hold symmetry has become highly modified. Echinoderms are all marine unable to tolerate low salinities. Many echinoderms use a hydraulic system called the water-vascular system to move around. This is a series of canals, off which branch small tube feet or podia. By contracting muscles in the tube foot the podia can be extended or contracted. The foot in many species ends in a sucker which helps the animal attach to the substrate. Only Asteroids, echinoids and holothurians will be dealt with here.
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