Segmented worm of the class Polychaeta, characterized by having a pair of fleshy paddles (parapodia) on each segment, together with prominent bristles (setae), and a well-developed head with a pair each of eyes, antennae, and palps. Most bristle-worms are marine, and live in burrows in sand and mud or in rock crevices and under stones. More than 5,300 species are recognized.

The bristle-worms can be conveniently divided into two loosely defined groups: the Errantia, which are free-living (that is swimming, crawling, or actively burrowing), and the Sedentaria, which live within a tube or a permanent burrow.

In many tube-dwelling worms such as Chaetopterus, the body is divided into different regions produced by variation in the size and function of the parapodia and the presence of extra gills for respiration. Most are filter-feeders and, like the peacock worm Sabella, possess a beautiful crown of tentacles which, when expanded, acts as a filter to trap the minute algae the worms feed on. Others, such as Terebella, use their tentacles to pick up detritus on the sand surface.

Classification
Bristle-worms are in class Polychaeta in phylum Annelida.