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 walls. Nemerteans can have from two to 250 pigment-cup eyes. The one on the left has two eyes, and on the right six eyes. There are two Classes; the Anopla with a simple proboscis; and the Enopla with a more complicated proboscis armed with stylets (see Prostoma rubrum right, which in some can deliver toxic secretions. The bootlace worm, Lineus sp., is perhaps one of the more commonly known Nemertea. It looks just like its name suggests, and can be as much as 10 metres long. It is usually found entangled under rocks and crevices, and easily breaks apart if you try to untangle it. Regeneration from the fragments is simple, and is an alternative means of reproduction for this worm. It can eat prey wider than itself, and wide prey shows up as a bulge in its body which gets smaller as digestion proceeds and the prey passes further down the body. Many are brightly coloured.