Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms . Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial forms; they are found in all marine habits, and throughout the world's oceans . Nemerteans are named for Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini and Nemertinea.
Most nemerteans are carnivorous and predatory, catching prey with their proboscis , although some are scavengers and some are herbivores . In some families, it is armed with a sharp stylet which may be poisonous, while those that lack the stylet often use a sticky secretion on the proboscis to entrap their prey. The proboscis is wrapped around the prey, which is normally other invertebrates such as crustaceans and annelids and can be many times larger than the nemertean itself, and the prey is then stabbed repeatedly with the stylet until dead . A few, such as Malacobdella, live parasitically in the mantle cavity of molluscs and live on the food filtered by their hosts .
It is claimed that some ribbon worms will eat themselves if no other food is available.
Nemertean worms are unique in possessing a "cerebral organ" — a sensory and regulatory organ closely associated with the brain .
Nemerteans often have numerous gonads, and most species have separate sexes, although all the freshwater forms are hermaphroditic. Fertilisation is usually external, although some species have both internal fertilisation and live birth .
The fossil record of the phylum is sparse, as expected for a group of soft-bodied animals, but even the hard stylets are not found. The only possible nemertean fossil is Archisymplectes from the Mazon Creek biota of the Pennsylvanian of Illinois .
The traditional classes of Enopla for nemerteans armed with one or more stylets and Anopla for those without are not monophyletic is not supported by molecular data . Similarly, the subclass Bdellonemertea, erected for nemerteans which live as parasites on molluscs, is nested within Hoplonemertea, and probably represents a specislised offshoot from that group rather than an independent lineage . Recent molecular phylogenetic study has, however, confirmed the monophyly of each of Heteronemertea and Hoplonemertea, as well as the expected paraphyly of Palaeonemertea .