Nemertea is a phylum of largely aquatic invertebrate animals also known as ribbon worms or proboscis worms and characterized by long, thin, unsegmented body that is flattened posteriorly and cylindrical anteriorly and has a long retractable proboscis that can evert for such purposes as capturing prey, defense, and locomotion. Most of the 1,400 or so species are marine, with a few living in freshwater and a small number of fully terrestrial forms. While the smallest reaches only 0.5 centimeters, the largest ones can reach 30 meters long, and reportedly even 50 meters in length, which would make it the world's longest animal.
Nemertea play an important ecological role in food chains. Most species are carnivorous, consuming invertebrates such as annelids, mollusks, crustaceans, jellyfish, and so forth, but also fish and fish eggs. They can be voracious predators, eating almost anything and animals much larger than themselves. Some also scavenge for food or are herbivores. In turn, they provide food for fish and larger invertebrates.
Nemertean worms are soft-bodied, unsegmented animals, typically with an elongate body that is long and thin, and distinguished by the presence of an eversible proboscis. The anterior portion of the body is cyclindrical and the posterior part is flattened (Smith 2008). Although generally considered acoelomate, the cavity which contains the proboscis includes a true coelom (Turbeville et al. 1992). The circulatory system of nemerteans is closed, as is the digestive system, which includes a separate mouth and anus (unlike flatworms, which have a single opening). The mouth is ventral. Body movements and contractions of the blood vessel walls drives the blood flow system (Smith 2008). The nervous system includes a brain and several nerve cords; nemerteans also have pigment-cup eyes, ranging from two to 250 such eyes depending on the species (Smith 2008). respiration is entirely by diffusion (Smith 2008).
Nemertean worms are unique in possessing a "cerebral organ"-a sensory and regulatory organ closely associated with the brain (Moore and Gibson 2001).
The proboscis, when retracted, sits in a dorsal cavity, separate from the digestive tract, that takes up most of the length of the worm. Muscular contraction causes pressure in the proboscis cavity and everts the proboscis. The action of a longitudinal muscle causes retraction. The proboscis serves for capturing prey, and can also be used in locomotion and defense (Smith 2008).
Nemerteans often have numerous gonads, and most species have separate sexes, although all the freshwater forms are hermaphroditic. Fertilization is usually external, although some species have both internal fertilization and live birth (Moore and Gibson 2001).
Some nemerteans, such as the bootlace worm (Lineus sp.) have exhibited regeneration, which offers another means of reproduction (Smith 2008).
Nemerteans range in size from 5 millimeters (0.2 inches) to over 30 meters (98 feet) long in the case of the European Lineus longissimus. There are also reports of specimens up to 50 or 60 meters (164-197 feet) long, which would make it the longest animal in the world (Telnes; Smith 2008); the longest vertebrate on record is a female blue whale, 29.9 meters (98 feet) long.
Nemerteans are named for Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini and Nemertinea.
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 (Gibson 1995).
Nemerteans are found in all marine habits and throughout the world's oceans (Moore and Gibson 2001). They are often found in shallow waters, 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 (Smith 2008). However, some are scavengers and some are herbivores (Shaner). A few, such as Malacobdella, live parasitically in the mantle cavity of mollusks and live on the food filtered by their hosts (Waggoner and Collins 2001).
Carnivorous nemerteans normally prey upon other invertebrates, such as crustaceans, annelids (such as polychaetes), mollusks, sponges, jellyfish, and so forth, but also are known to eat fish eggs and fish. They can be voracious predators and consume prey animals many times larger than the nemertean itself. In some families, the nemertean is armed with a sharp stylet, which may be poisonous. The proboscis is wrapped around the prey and the prey is then stabbed repeatedly with the stylet until dead (Waggoner and Collins 2001). Those that lack the stylet often use a sticky secretion on the proboscis to entrap their prey.
The earliest record of a nemertean worm is probably an account by Olaus Magnus in 1555 of a long, grayish-blue marine worm, which is probably Lineus longissimus. However, the first formal description of a species of Nemertea did not happen until Gunnerus described the same species (as Ascaris longissima) in 1770 (Gibson 1995). Once classified as "degenerate" flatworms, nemerteans are now recognized as a separate phylum, more closely related to higher, coelomate phyla in Lophotrochozoa, such as Annelida and Mollusca (TOL 2002). The phylum has also been known as Rhyncocoela.
By 1995, a total of 1,149 species had been described, and grouped into 250 genera (Gibson). Traditionally nemerteans have been arranged into two classes, Anopla and Enopla. Members of Anopla have a simple proboscis and members of Enopia have a more complex proboscis armed with stylets (Smith 2008).
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 (Waggoner and Collins 2001).
The traditional classes of Enopla, for nemerteans armed with one or more stylets, and Anopla, for those without, are not monophyletic, as monophyly is not supported by molecular data (Sundberg et al. 2001). Similarly, the subclass Bdellonemertea, erected for nemerteans that live as parasites on mollusks, is nested within Hoplonemertea, and probably represents a specialized offshoot from that group rather than an independent lineage (Sundberg et al. 2001). Recent molecular phylogenetic study has, however, confirmed the monophyly of each of Heteronemertea and Hoplonemertea subclasses, as well as the expected paraphyly of the subclass Palaeonemertea (Thollesson and Norenburg 2003).
- Integrated Taxonomic Information System (ITIS). 1999. Nemertea. ITIS Taxonomic Serial No.: 57411. Retrieved December 20, 2008.
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- Telnes, K. n.d. Giant ribbon worm. The Marine Fauna Gallery of Norway. Retrieved December 20, 2008.
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- Tree of Life Web Project (TOL). 2002. Bilateria. Triploblasts, bilaterally symmetrical animals with three germ layers. Tree of Life Web Project version January 1, 2002. Retrieved December 20, 2008.
- Turbeville, J. M., K. G. Field, and R. A. Rafl. 1992. Phylogenetic position of Phylum Nemertini, inferred from 18s rRNA sequences: molecular data as a test of morphological character homology. Molecular Biology and Evolution 9(2): 235-249.
- Waggoner, B., and A. G. Collins. 2001. Introduction to the Nemertini: Tied up in knots. University of California Museum of Paleontology. Retrieved December 20, 2008.