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any large, segmented marine worm of the family Sabellidae (class Polychaeta, phylum Annelida). The name is also occasionally applied to members of the closely related family Serpulidae. Sabellids live in long tubes constructed of mud or sand cemented by mucus, whereas serpulids build tubes of calcareous materials. The epithet feather-duster refers to the multicoloured crown of finely divided tentacles that are attached in two groups, one on either side of the worm's head. The worm extends the tentacles for feeding and breathing but quickly retracts them at the first sign of danger.