Pomatoceros triqueter is a species of tube-building annelid worm in the class Polychaeta. It is common on the north eastern coasts of the Atlantic Ocean and in the Mediterranean Sea. Polychaetes, or marine bristle worms, have elongated bodies divided into many segments. Each segment may bear setae (bristles) and parapodia (paddle-like appendages). Some species live freely, either swimming, crawling or burrowing, and these are known as "errant". Others live permanently in tubes, either calcareous or parchment-like, and these are known as "sedentary".