About 1200 species of nemerteans are known. Most are free-living, most are marine and are found in just about every marine habitat, including the pelagic realm. The diagnostic feature of nemerteans is an eversible muscular proboscis that can be longer than the body and lies in a closed, fluid-filled chamber, the rhynchocoel. A few are supralittoral and terrestrial, especially in the tropics, and a few live in freshwater. A very marine forms have obligate associations with other animals; most noteworthy among these are the Malacobdellidae, which live in the mantle cavity of certain bivalves and filter food from the host, and Carcinonemertidae, which live on decapod crustacea and prey on encapsulated developing embryos carried by female hosts. Most hoplonemerteans are obligate predators, often on crustaceans, but also annelids and mollusks. Most palaeonemerteans and pilidiophorans may preferentially be predators but often will scavenge as well. If food cannot be swallowed whole, then it often is consumed by exodigestion and suctorial feeding. Nemerteans often capture prey significantly larger than themselves and often can stretch the mouth and boday to swallow prey several times their own resting diameter.
Nemerteans have a fully ciliated, highly glandular epidermis, and usually a highly muscular body. Benthic forms are capable of extreme elongation and contraction, enabling them to squeeze into exceedingly tight crevices and pores. Hence, most tend to be difficult to find or collect. Although some nemerteans have distinctive color patterning that can be diagnostic, at least for a given region, most are best described as relatively drab or monotonic and lack convenient features to diagnose them to species. While some instances of color variation are known, in most instances the risk is that similarly colored nemerteans of a particular taxonomic group from different locales are different species.
Traditional classification recognized two classes that could be conveniently diagnosed by presence or absence of a ventral mouth; this is no longer the case (Thollesson & Norenburg 2003). What may be referred to as the orders Palaeonemertea and Pilidiophora have a ventral mouth, whereas the order Hoplonemertea has the mouth internally combined with the rhynchodeum, the internal head passage through which the proboscis is everted to the outside. Hence, a common pore serves as the mouth and proboscis pore.