About 100 species have so far been described, all less than 1 mm long. The most characteristic feature is the segmented, chitinous, spiny cuticle (see Echinoderes sp. right). They are usually found burrowing through mud and silt in shallow waters. They burrow by everting a retractable stylets that has spines towards the end; these attach to the surrounding sediment allowing the animal to drag its body along. The stylets is everted by the pressure exerted when the muscles flatten the body by pulling the dorsal and ventral plates together. The head can be retracted into the trunk leaving just the ring of spiny protective plates. They feed on diatoms and edible particles found in the sediments while burrowing.