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.