These are commonly known as the ribbon worms or proboscis worms. They range in length from less than 0.5 cm to over 50 m. This is more than twice the average length of an adult blue whale. So at 50 m the ribbon worm is the world's longest animal.

There are about 1200 known species. They are found mainly in shallow waters, beneath shells, stones, amongst algae, and burrowing in sand and mud.They are acoelomate carnivores using their eversible proboscis to grasp prey. The proboscis lies in a cavity that runs almost the entire body length (see Prostoma rubrum right), as well as catching prey it can be used in locomotion and in defence. Contraction of the muscles exerts pressure in the proboscis cavity causing the proboscis to evert; it is retracted by the action of a longitudinal muscle. The blood flow system is driven by body movements and the contractions of the blood vessel