Most Nemerteans are dioecious (having two separate sexes) free living organisms that like to spend much of their time beneath rocks or thick algal growth or else burrowing into soft mud or sand. Though most of the free living species live on the sea floor near the coast, some have been recorded from depths as great as 4000m and there are a number of genera adapted to live permanently swimming at sea where they feed on other planktonic organisms. The 4 species in the Genus Malacobdella live commensally in the mantle cavity of bivalve molluscs, they do not feed on mollusc tissue but as far as is known the molluscs gain no advantage from the Nemerteans presence. Various species are also commonly found in the pharyngeal cavity of tunicates. The 4 or 5 species in the genus Carcinonemertes are parasitic on crabs, usually female crabs. Otherwise Nemerteans are mostly carnivores or scavengers on recently dead animal remains. The commensal Malacobdella species are filter feeders on small organisms that enter their hosts/partners mantel cavity with the inflow of water. Thus it can be seen that Nemerteans occupy a wide range of habitats.
[Species of animal that live in the open oceans are called 'marine pelagic species' while those that live on the sea floor near the coast are called 'littoral marine species'.]