Nemertea is a
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
of
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s also known as
ribbon worms or
proboscis worms.
Most of the 1,400 or so
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
are marine, with a few living in
fresh waterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
and a small number of
terrestrial formsTerrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats . Terrestrial animals evolved from marine animals...
; they are found in all marine habits, and throughout the world's oceans. Nemerteans are named after Nemertes, one of the
NereidsIn Greek mythology, the Nereids are sea nymphs, the fifty daughters of Nereus and Doris. They often accompany Poseidon and are always friendly and helpful towards sailors fighting perilous storms. They are particularly associated with the Aegean Sea, where they dwelt with their father in the...
of
Greek mythologyGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
, and alternative spellings for the phylum have included
Nemertini and
Nemertinea. Libbie HymanLibby Henrietta Hyman , was an American zoologist.Born in Des Moines, Iowa, she was the daughter of Joseph Hyman and Sabina Neumann. Hyman's father, a Polish/Russian Jew, adopted the surname when he immigrated to the United States as a youth...
named them Rhynchocoela, a name used primarily in North America but gradually abandoned since the 1980s.
History
The earliest record of a nemertean worm is probably an account by
Olaus MagnusOlaus Magnus was a Swedish ecclesiastic and writer, who did pioneering work for the interest of Nordic people. He was reported as born in October 1490 in Östergötland, and died on August 1, 1557. Magnus, Latin for the Swedish Stor “great”, is a Latin family name taken personally, and not a...
in 1555 of a long, greyish-blue marine worm, which is probably
Lineus longissimusThe bootlace worm is in the phylum Nemertea or ribbon worms. It is one of the longest animals known, with specimens up to 30 m long being reported and some speculation that they may grow as long as 60 m, which would make it the longest animal in the world. They are however usually only 5 to 10 mm...
, but the first species was not formally described until
GunnerusJohan Ernst Gunnerus was a Norwegian bishop and botanist.Gunnerus was born at Christiania. He was bishop of Trondheim from 1758, and professor of theology at the university of Copenhagen.- Biography :...
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.
Ecology and distribution
The majority of nemertean worms live on or in the sea floor, with many species extending into
brackish waterBrackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salten" or "salty"...
in estuaries, and some
freshwaterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
or fully
terrestrialTerrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats . Terrestrial animals evolved from marine animals...
species. They are often found in and among
seaweedSeaweedSeaweed has antioxidents. Is a loose colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...
s,
rockIn geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s,
musselThe common name mussel is used for members of several families of clams or bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
and
barnacleA barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
beds, or buried in mud, sand, or gravel substrates. Freshwater genera include the large genus
Prostoma, while the terrestrial forms are best represented by
Geonemertes, a genus mostly found in
AustralasiaAustralasia is a region of Oceania: Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes . He derived it from the Latin for "south of Asia" and differentiated the...
, but with one species in the
SeychellesSeychelles , officially the Republic of Seychelles , is an archipelago nation of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar...
, one found widely across the
Indo-PacificThe Indo-Pacific is a biogeographic region of the earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...
, one from
Tristan da CunhaTristan da Cunha is a remote volcanic group of islands in the south Atlantic Ocean. It is the most remote inhabited archipelago in the world, lying from the nearest land, South Africa, and from South America. It is part of the British overseas territory of Saint Helena, Ascension and Tristan da...
in the South Atlantic, and one,
G. chalicophora, first found in the
PalmengartenThe Palmengarten is one of two botanical gardens in Frankfurt am Main, Hesse, Germany, and is located in the city district Westend-Nord. It is the largest garden of its kind in Germany....
in
FrankfurtFrankfurt am Main , commonly known simply as Frankfurt, is the largest city in the German state of Hesse and the fifth-largest city in Germany, with a 2008 population of 670,000. The urban area had an estimated population of 2.26 million in 2001...
, but since discovered in the
Canary IslandsThe Canary Islands are a Spanish archipelago which, in turn, forms one of the Spanish Autonomous Communities and an Outermost Region of the European Union. The archipelago is located just off the northwest coast of mainland Africa, 100 km west of the disputed border between Morocco and the...
,
MadeiraMadeira is a Portuguese archipelago in the mid Atlantic Ocean that lies between and . It is one of the Autonomous regions of Portugal, with Madeira Island and Porto Santo Island being the only inhabited islands...
and the
AzoresThe Azores is a Portuguese archipelago in the Atlantic Ocean, about from Lisbon and about from the east coast of North America. The two westernmost Azorean islands actually lie on the North American plate...
.
Most nemerteans are carnivorous and predatory, catching prey with their
proboscisIn general, a proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate..-Etymology:...
, although some are
scavengerScavenging, or necrophagy, is a carnivorous feeding behaviour in which a predator consumes corpses or carrion that were not killed to be eaten by the predator or others of its species. Scavengers play an important role in the ecosystem by contributing to the decomposition of dead animal remains...
s and some are
herbivoreA herbivore is an animal that is adapted to eat plants and not meat.Herbivory is a form of predation in which an organism consumes principally autotrophs such as plants, algae and photosynthesizing bacteria....
s. 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
crustaceanCrustaceans are a very large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles...
s and
annelidThe annelids, collectively called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments...
s 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
commensallyIn ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism and parasitism .Commensalism derives from the English word commensal, meaning "sharing of food" in human social...
in the
mantle cavityThe mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass....
of molluscs by "stealing" from the food filtered by the host.
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
coelomThe coelom is a fluid filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
. The
circulatory system- [Headline text]--75.137.171.242 01:21, 21 October 2009 :bThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, nitrogen waste products, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pHb to maintain...
of nemerteans is closed, as is the digestive system, which includes separate
mouthThe mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva...
and
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food...
(unlike
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, which have a single opening). The
nervous systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
includes a
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
and several nerve cords;
respirationIn animal physiology, respiration is the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
is entirely by
diffusionMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
.
Nemertean worms are unique in possessing a "cerebral organ" — a sensory and regulatory organ closely associated with the brain.
Nemerteans often have numerous
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s, and most species have separate sexes, although all the freshwater forms are
hermaphroditicIn a biological context, a hermaphrodite is an animal or plant that has both male and female reproductive organs.Many taxonomic groups of animals , do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both partners can...
. Fertilization is usually
externalExternal fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
, although some species have both
internal fertilizationInternal fertilization is a form of fertilization of an egg by within the body of an animal, whether female or hermaphroditic. This is distinct from external fertilization, where the union of the ova and spermatozoa occur outside of the organism....
and
live birthA live birth occurs when a fetus, whatever its gestational age, exits the maternal body and subsequently shows any sign of life, such as voluntary movement, heartbeat, or pulsation of the umbilical cord, for however brief a time and regardless of whether the umbilical cord or placenta are...
.
Length
Nemerteans range in size from 5 mm to over 30 metres long in the case of the European
Lineus longissimusThe bootlace worm is in the phylum Nemertea or ribbon worms. It is one of the longest animals known, with specimens up to 30 m long being reported and some speculation that they may grow as long as 60 m, which would make it the longest animal in the world. They are however usually only 5 to 10 mm...
. 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 whaleThe Blue Whale is a marine mammal belonging to the suborder of baleen whales . At up to in length and or more in weight, it is the largest animal ever known to have existed....
, 29.9 m long.
Classification
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
PennsylvanianThe Pennsylvanian is in the ICS geologic timescale the youngest subperiod or upper subsystem of the Carboniferous period. It lasted from roughly to Ma . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the...
of
IllinoisIllinois , the 21st state admitted to the United States of America, is the most populous and demographically diverse Midwestern state and the fifth most populous state in the nation...
.
Once classified as "degenerate"
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, nemerteans are now recognised as a separate phylum, more closely related to higher, coelomate phyla in
LophotrochozoaThe Lophotrochozoa are a major grouping of protostome animals. The taxon was introduced in 1995 in a paper by Kenneth M Halanych et al. based on molecular data...
, such as Annelida and
MolluscaMolluscs
[Spelled mollusk in the USA, see reasons given in Rosenberg's ; for the spelling "mollusc" see the reasons given by .] are animals belonging to the phylum Mollusca. There are around 93,000 recognized extant species, making it the largest marine phylum with about 23% of...
. The sequence of the genome of the mitochondrion of
Cephalothrix simula places this phylum closer to the coelomate lophotrochozoans rather than the acoelomate platyhelminths consistent with earlier work.
The traditional classes of
EnoplaEnopla is one of the classes of the worm phylum Nemertea, characterized by the presence of a peculiar armature of spines or plates in the proboscis.-Evolution and systematics:...
for nemerteans armed with one or more stylets and
AnoplaAnopla are a class of marine worms of the phylum Nemertea, characterized by the absence of stylets on the proboscis, the mouth being below or behind the brain, and by having separate openings for the mouth and proboscis. The other class of Nemertea are the Enopla. Although Anopla is a...
for those without are not
monophyleticIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...
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 specialised offshoot from that group rather than an independent lineage . Recent
molecular phylogeneticMolecular phylogenetics, also known as molecular systematics, is the use of the structure of molecules to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree....
study has, however, confirmed the
monophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...
of each of
HeteronemerteaHeteronemertea is a monophyletic order of about 500 species, containing genera such as Lineus and Cerebratulus and including the largest and most muscular nemerteans. Almost all heteronemerteans have three primary body-wall muscle strata: an outer longitudinal, a middle circular, and an inner...
and Hoplonemertea, as well as the expected
paraphylyIn phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all the descendants of that ancestor.- Relation to monophyletic groups :...
of
PalaeonemerteaPalaeonemertea may be a para- or polyphyletic Order, consisting of 3 to 5 clades and totalling about 100 species. These worms have several apparently simple features and, as their name suggests, they are often considered to be the most primitive nemerteans. The primary body-wall musculature...
.