A small group of unsegmented, wormlike, marine animals once linked with the Sipuncula and Priapulida under the taxon Gephyrea. Since 1940, echiurans have been regarded as a separate phylum of the animal kingdom with affinities to the segmented annelid worms. They range from the tropical to the polar seas, burrowing in sea-floor sediments from the intertidal area to depths of 30,000 ft (9140 m).