Polychaeta is a class of the phylum Annelida or segmented worms, perfectly metameric, with cylindrical body segments, a pair of fleecy appendages called parapodia, with many chaetae. It is a vast class containing most of the marine annelids. They can be free moving or errant, or sedentary living in tubes. Some live in the pelagic, others in gravel or sand, or in burrows in muddy sediments. They show many varieties of feeding and body forms. See Annelids.