The nemertines or nemerteans are a small group of elongated, unsegmented, soft-bodied worms. Most of the species are marine. They are common inhabitants of shallow-water environments, crawling over solid substrata, burrowing into sediment, or lurking under stones, rocks or mats of algae. Few members of this phylum are found in freshwater or are terrestrial; these species are most likely to descend from marine ancestors. Many marine nemertine species live commensally with invertebrates from other phyla, particularly the Arthropoda and Mollusca, but only a few parasitic nemertines are known.
Nemertines are ciliated externally and secrete mucus through which they progress. Small nemertines can move over substrata exclusively by means of ciliary beating. Larger specimens often generate waves of muscle contraction for locomotion over hard substrata or through soft sediment. A few species can swim by generating relatively violent waves of muscular contraction.