Nemertea

Nemertea Temporal range: Middle Triassic–recent PreꞒ Ꞓ O S D C P T J K Pg N Possible Ordovician and Carboniferous records
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
(unranked):	Spiralia
Superphylum:	Lophotrochozoa
Phylum:	Nemertea Schultze , 1851
Classes
See text
Synonyms [1]
Nemertini Nemertinea Rhynchocoela

Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species.^[2][3] Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below the midline of the body, the

In 1555 Olaus Magnus wrote of a marine worm which was apparently 17.76 metres (58.3 ft) long ("40 cubits"), about the width of a child's arm, and whose touch made a hand swell. William Borlase wrote in 1758 of a "sea long worm", and in 1770 Gunnerus wrote a formal description of this animal, which he called Ascaris longissima. Its current name, Lineus longissimus, was first used in 1806 by Sowerby.^[4] In 1995, a total of 1,149 species had been described and grouped into 250 genera.^[5]

Nemertea are named after the Greek sea-

The typical nemertean body is very thin in proportion to its length.[9] The smallest are a few millimeters long,^[10] most are less than 20 centimetres (7.9 in), and several exceed 1 metre (3.3 ft). The longest animal ever found, at 54 metres (177 ft) long, may be a specimen of Lineus longissimus,^[9] Ruppert, Fox and Barnes refer to a Lineus longissimus 54 metres (177 ft) long, washed ashore after a storm off St Andrews in Scotland.^[11] Other estimates are about 30 metres (98 ft).^[12] Zoologists find it extremely difficult to measure this species.^[13] For comparison:

• The longest recorded blue whale was 33.58 metres (110.2 ft).^[14]
• The dinosaurs Argentinosaurus and Patagotitan are estimated at approximately 35 metres (115 ft) and 31 metres (102 ft) respectively.^[15]
• A specimen of the Arctic giant jellyfish
  Cyanea capillata arctica was 36.5 metres (120 ft) long.^[16]

L. longissimus, however, is usually only a few millimeters wide.[17] The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species,^[17][9] but reduce their length to 50% and increase their width to 300% when disturbed.^[12] A few have relatively short but wide bodies, for example Malacobdella grossa is up to 3.5 centimetres (1.4 in) long and 1 centimetre (0.39 in) wide,^[9][18] and some of these are much less stretchy.^[17] Smaller nemerteans are approximately cylindrical, but larger species are flattened dorso-ventrally. Many have visible patterns in various combinations of yellow, orange, red and green.^[9]

The outermost layer of the body has no

They are unsegmented, but at least one species, Annulonemertes minusculus, is segmented. But this is assumed to be a derived trait. The segmentation does not include the coelom and body wall, and is therefore referred to as pseudosegmentation.[20]^[21]

The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and the

The proboscis is an infolding of the body wall, and sits in the rhynchocoel when inactive.^[10] When muscles in the wall of the rhynchocoel compress the fluid inside, the pressure makes the proboscis jump inside-out along a canal called the rhynchodeum and through an orifice, the proboscis pore. The proboscis has a muscle which attaches to the back of the rhynchocoel, can stretch up to 30 times its inactive length and acts to retract the proboscis.^[9]

The proboscis of the class Anopla exits from an orifice which is separate from the mouth,^[9] coils around the prey and immobilizes it by sticky, toxic secretions.^[22] The Anopla can attack as soon as the prey moves into the range of the proboscis.^[23] Some Anopla have branched proboscises which can be described as "a mass of sticky spaghetti".^[9] The animal then draws its prey into its mouth.^[10]

In most of the class Enopla, the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has a stylet, a calcareous barb,^[9] with which the animal stabs the prey many times to inject toxins and digestive secretions. The prey is then swallowed whole or, after partial digestion, its tissues are sucked into the mouth.^[22] The stylet is attached about one-third of the distance from the end of the everted proboscis, which extends only enough to expose the stylet. On either side of the active stylet are sacs containing back-up stylets to replace the active one as the animal grows or an active one is lost.^[9] Instead of one stylet, the Polystilifera have a pad that bears many tiny stylets, and these animals have separate orifices for the proboscis and mouth, unlike other Enopla.^[24][25] The Enopla can only attack after contacting the prey.^[23]

Some nemerteans, such as L. longissimus, absorb organic food in solution through their skins, which may make the long, slim bodies an advantage.

In the simplest type of circulatory system, two lateral vessels are joined at the ends to form a loop. However, many species have additional long-wise and cross-wise vessels. There is no heart nor pumping vessels,[30] and the flow of fluid depends on contraction of both the vessels and the body wall's muscles. In some species, circulation is intermittent, and fluid ebbs and flows in the long-wise vessels.^[27] The fluid in the vessels is usually colorless, but in some species it contains cells that are yellow, orange, green or red. The red type contain hemoglobin and carry oxygen, but the function of the other pigments is unknown.^[27]

Excretion

Nemertea use organs called

The

Paranemertes peregrina, which feeds on polychaetes, can follow the prey's trails of mucus, and find its burrow by backtracking along its own trail of mucus.^[22]

Movement

Nemerteans generally move slowly,^[10] though they have occasionally been documented to successfully prey on spiders or insects.^[37] Most nemerteans use their external cilia to glide on surfaces on a trail of slime, some of which is produced by glands in the head. Larger species use muscular waves to crawl, and some aquatic species swim by dorso-ventral undulations. Some species burrow by means of muscular peristalsis, and have powerful muscles.^[9] Some species of the suborder Monostilifera, whose proboscis have one active stylet, move by extending the proboscis, sticking it to an object and pulling the animal toward the object.^[24]

Reproduction and life-cycle

Larger species often break up when stimulated, and the fragments often grow into full individuals. Some species fragment routinely and even parts near the tail can grow full bodies.

The species Paranemertes peregrina has been reported as having a life span of around 18 months.^[34]

Ecological significance

Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or the

Near San Francisco the nemertean Carcinonemertes errans has consumed about 55% of the total egg production of its host, the dungeness crab Metacarcinus magister. C. errans is considered a significant factor in the collapse of the dungeness crab fishery.^[23] Other coastal nemerteans have devastated clam beds.^[9]

The few predators on nemerteans include bottom-feeding fish, some sea birds, a few invertebrates including horseshoe crabs, and other nemerteans.^[9] Nemerteans' skins secrete toxins that deter many predators, but some crabs may clean nemerteans with one claw before eating them.^[28] The American Cerebratulus lacteus and the South African Polybrachiorhynchus dayi, both called "tapeworms" in their respective localities, are sold as fish bait.^[9]

Taxonomy

Traditional taxonomic classification has divided the group into two classes and four orders:

• Class Anopla ("unarmed"). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.^[24]
  • burrowing by peristalsis.^[24]
  • Order Heteronemertea. Comprises about 400 species. The majority are marine, but three are freshwater. Their body-wall muscles are disposed in four layers, alternately circular and length-wise starting from the outermost layer. The order includes the strongest swimmers. Two genera have branched proboscises.^[24]
• Class
  Bdellonemertea. Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.^[24]
  • Order
    filter feed and all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a sucking pharynx and a posterior stucker, with which they move like inchworms.^[24]
  • Order
    pelagic sea water, in freshwater and on land. They feed by commensalism and parasitism, and are armed with stylet(s)^[24]
    • Suborder Monostilifera. Includes 500 species with a single central stylet. Some use the stylet for locomotion as well as for capturing prey.^[24]
    • Suborder Polystilifera. Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets.^[24]

Recent molecular phylogenetic studies divided the group into two superclasses, three classes, and eight orders:^[45]

• Superclass Pronemertea
  • Class Palaeonemertea
    • Order Carinomiformes
    • Order Tubulaniformes
    • Order Archinemertea
• Superclass Neonemertea
  • Class Pilidiophora
    • Order Hubrechtiiformes
    • Order Heteronemertea
  • Class Hoplonemertea (= Enopla)
    • Order Polystilifera
    • Order Monostilifera (includes Bdellonemertea)
• incertae sedis
  • Order Arhynchonemertea (provisionally has been separated its own class Arhynchocoela in 1995)

Evolutionary history

Fossil record

As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare.^[10][44] Knaust (2010) reported nemertean fossils and traces from the Middle Triassic of Germany.^[46] One might expect the stylet of a nemertean to be fossilized, since it is made of the mineral calcium phosphate, but no fossilized stylets have been found.^[10][44]

The

Knaust & Desrochers (2019) reported fossils of vermiform organisms with a wide range of morphologies occurring on bedding planes from the Late Ordovician (Katian) Vauréal Formation (Canada). In the specimens preserving the anterior end of the body, this end is pointed or rounded, bearing a rhynchocoel with the proboscis, which is characteristic for nemerteans. The authors attributed these fossils to nemerteans and interpreted them as the oldest record of the group reported so far. However, Knaust & Desrochers cautioned that partly preserved putative nemertean fossils might ultimately turn out to be fossils of turbellarians or annelids.^[48]

It has been suggested that

Within Nemertea

Nemertea	Palaeonemertea Heteronemertea Enopla Bdellonemertea Hoplonemertea Monostilifera Polystilifera

Groups within Nemertea, by Ruppert, Fox and Barnes (2004).

paraphyletic.^[51]

There is no doubt that the phylum Nemertea is

synapomorphies (trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel.^[53]

While Ruppert, Fox and Barnes (2004) treat the Palaeonemertea as monophyletic,

paraphyletic and basal (contains the ancestors of the more recent clades).^[53] The Anopla ("unarmed") represent an evolutionary grade of nemerteans without stylets (comprising the Heteronemertea and the Palaeonemerteans), while Enopla ("armed") are monophyletic, but find that Palaeonemertea is doubly paraphyletic, having given rise to both the Heteronemertea and the Enopla.^[51][53] Ruppert, Fox and Barnes (2004) treat the Bdellonemertea as a clade separate from the Hoplonemertea,^[51] while Thollesson and Norenburg (2003) believe the Bdellonemertea are a part of the Monostilifera (with one active stylet), which are within the Hoplonemertea – which implies that "Enopla" and "Hoplonemertea" are synonyms for the same branch of the tree.^[53] The Polystilifera (with many tiny stylets) are monophyletic.^[51][53]

Relationships with other phyla

English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre-

entoproct larvae. The protonephridia of nemertea and flatworms are different in structure,^[54] and in position – the flame cells of nemertea are usually in the walls of the fluid vessels and are served by "drains" from which the wastes exit by a small number of tubes through the skin,^[27] while the flame cells of flatworms are scattered throughout the body.^[55] Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia.^[54]

According to more recent analyses, in the development of nemertean embryos, ecto

triclads, but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among the Spiralia (animals in which the early cell divisions make a spiral pattern) such as sipunculans, echiurans and many annelids.^[54]

Bilateria	Acoelomorpha (Acoela and Nemertodermatida) Deuterostomia (Echinoderms, chordates , etc.) Protostomia priapulids , etc.) Lophotrochozoa Bryozoa Annelida and phyla merged into them Mollusca Brachiopoda Nemertea Dicyemida Myzostomida Platyzoa Platyhelminthes Gastrotricha Other Platyzoa

Relationships of Nemertea to other Bilateria:^{[56][Note 1]}

Nemerteans' affinities with Annelida (including Echiura,

molecular phylogeny, which compares sections of organisms DNA and RNA. While analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear.^[56][58]

Most

Echinodermata and Chordata. The Acoelomorpha, which are neither protostomes nor deuterostomes, are regarded as basal bilaterians.^[56][58][59]

See also

• Emplectonema neesii
• Lineus longissimus
• Parborlasia corrugatus

Notes

1. ^ Sipuncula were merged into Annelida in 2007.^[57]

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External links

Wikisource has the text of the 1911 Encyclopædia Britannica article "Nemertina".

• The Marine Biological Laboratory: Phylum Nemertea (Nemertinea, Nemertini, Rhynchocoela)
• Nemertea LifeDesk
• Video of a Nemertea in Puget Sound