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Errantia (scaleworms, sea mice; phylum Annelida, class Polychaeta) A subclass of polychaete worms which have a large number of body segments, those of the head and posterior differing from the rest. The mouth often has several paired jaws. Most are vagile predators, others adopt a burrowing mode of life. They are first recorded from the Cambrian. There is no acceptable ordinal classification for the Polychaeta; the class is divided into Errantia and Sedentaria for convenience but genera belonging to each subclass may not be related.