Nemertea

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, Nemertinea and Nemertea.

Ecology and distribution
The majority of nemertean worms live on or in the sea floor, with many species extending into brackish water in estuaries, and some freshwater or fully terrestrial species. Freshwater genera include the large genus Prostoma, while the terrestrial forms are best represented by Geonemertes, a genus mostly found in Australasia, but with one species in the Seychelles, one found widely across the Indo-Pacific, one from Tristan da Cunha in the South Atlantic, and one, G. chalicophora, first found in the Palmengarten in Frankfurt, but since discovered in the Canary Islands, Madeira and the Azores.

They are often found in and among seaweeds, rocks, mussel and barnacle beds, or buried in mud, sand, or gravel substrates.

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.

Anatomy
Nemertean worms are long, thin, unsegmented animals. They are distinguished by the presence of an eversible proboscis which is used for catching prey. Although generally considered acoelomate, the cavity which contains the proboscis includes a true coelom. The circulatory system of nemerteans is closed, as is the digestive system, which includes separate mouth and anus (unlike flatworms, which have a single opening). The nervous system includes a brain and several nerve cords; respiration is entirely by diffusion.

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.

Length
Nemerteans range in size from 5 mm to over 30 metres long in the case of the European Lineus longissimus. There are also reports of specimens up to 50 m or 60 m long, which would make it the longest animal in the world ; the longest vertebrate on record is a female blue whale, 29.9 m long.

Classification
The earliest record of a nemertean worm is probably an account by Olaus Magnus in 1555 of a long, greyish-blue marine worm, which is probably Lineus longissimus, but the first species was not formally described until Gunnerus described the same species (as Ascaris longissima) in 1770. In 1995, a total of 1,149 species had been described, and grouped into 250 genera.

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.

Once classified as "degenerate" flatworms, nemerteans are now recognised as a separate phylum, more closely related to higher, coelomate phyla in Lophotrochozoa, such as Annelida and Mollusca.

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.