The Polychaeta is the largest class of the phylum Annelida - the segmented worms. Polychaetes are highly distinct from other worms. Each body segment often bears a pair of parapodia with stiff bristles known as setae protruding to the side. Setae are used in burrowing, crawling and swimming and some species harbour stinging poisons within their setae.

The heads of polychaetes are very morphologically diverse. Some, such as carnivorous worms, have a specialised proboscis which can evert to seize prey. The heads of mobile polychaetes also have a vast array of sensory organs, including eyes and sensitive tentacles. Others such as sessile filter-feeding polychaetes have a reduced sensory capacity but have long feathery extensions that are often ciliated and collect suspended food and passing plankton from the water column.

Numerous deep-sea studies have found polychaete worms to be among the most diverse and abundant organisms collected. In general, they are small-bodied, with a reduced number of segments compared to shallow water species.

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