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Ribbon Worms

Nemertea

Notes

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General literature on nemerteans with ID-keys

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STRAND M. ,SAMUELSSON H. & SUNDBERG P. 2010. Nationalnyckeln: Stjärnmaskar-Slemmaskar. Förlag: Artdatabanken, SLU, Uppsala. App 80 species (Swedish) of nemerteans and sipunculans with Key facts in English.
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Malin Strand (strandmalin)
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Comprehensive Description

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The smallest of these soft, elongated, mostly marine worms may be threadlike and only a fraction of an inch long. The giants of the group, however, are the longest, though certainly not among the largest, of invertebrates. Exactly how long it is difficult to say, for all the ribbon worms are highly elastic, and the really long ones stretch out, threadlike, for yards and yards — some say much more than 30 yards in Lineus longissimus, the blackish brown worm of the North Sea. The English call it the "bootlace worm." Modest length, not more than about 8 inches, is more usual. The body may be cylindrical, as in Lineus,though more often flattened on both sides or flattened below and convex above.

Bright colorings of orange, red, purple, or green, these mostly on the upper surfaces, may betray the worms to the eyes of naturalists scanning rocky crevices or overturned stones at low tide. More often the colors blend with red or green algae or other colorful growths among which the worms live. To find small nemerteans, collectors place masses of seaweed or of bryozoan colonies that resemble delicate seaweed in dishes of sea water and let the small worms creep out on the walls of the dishes, where they can easily be seen. Some worms are white or yellowish, others somber grays or browns, but many are handsomely patterned with strongly contrasting rings or longitudinal stripes or both. The front end is not set off as a distinct head, though the tip may be expanded and have colored markings, several or numerous eyes, and sensory grooves, which make it look superficially like a head. The rear end is more or less pointed.

Another common name, proboscis worm, less widely used, calls attention to the most distinctive feature of nemerteans. This is a long, extensible, tubular proboscis that can be shot out the front end with explosive force to grasp prey or discourage enemies. The proboscis coils about the prey, holding it firmly and entangling it in sticky mucus which may be irritating or even poisonous. The proboscis is also everted as a device for burrowing in sand or mud or for attaching to objects as an aid in creeping about. It can be made to evert by irritating the animal, by plunging it into fresh water, or by placing it in a small dish of sea water and cautiously adding alcohol, drop by drop. The accurate aim of the proboscis receives recognition in the technical name of the phylum, Nemertea, from a Greek word that means "unerring." In some of the commonest worms the tip of the proboscis is armed with a sharp spike or stylet, which pierces the prey, sometimes several times, before a toxic secretion is poured on. Worms may have two or more pouches with a reserve supply of stylets, so that replacement can be made quickly if the main one is damaged. When not in use the proboscis is sheathed in a muscular tube that lies above the digestive tract.

As it goes on mostly at night, feeding is not often observed. The favored food seems to be annelids, and these have been seen to be swallowed whole, making a prominent bulge in the thin, elastic body of the nemertean. Mollusks, crustaceans, and fishes are also eaten, though bigger prey may be sucked at, not downed in one piece. Undigested residues do not have to be cast out the mouth, for the nemerteans are the lowest animals that have an anus, a second open- ing to the digestive tract, which voids materials from the rear end of the animal. The ribbon worms are built much like flatworms, but aside from the anus they can boast another important improvement. They have contractile blood vessels. Waves of contraction in the strong muscles of the body wall also help to push blood and food along their respective tubes, and in a worm at rest it is these powerful muscular waves that are seen to pass along the body.

A few ribbon worms swim by undulations of the long body. The young and the smaller forms glide along, by means of beating cilia on the body surface, over a lubricating bed of secreted slime. In larger worms more use is made of muscular contractions for creeping. Some even spiral ahead at times by agile body contortions.

One may grasp several inches of a delicate, slimy nemertean and pull cautiously lest it break, yet have it slip from one's fingers and disappear down a crack in the rock. Worms that do break in escaping from would-be captors, human or animal, almost always replace a missing rear end; and certain species can regenerate a whole worm from any fragment that contains a portion of one of the lateral nerve cords. As in flatworms, the capacity for regeneration goes with the natural capacity of certain species for reproducing asexually by fragmentation of the body, especially during warm months. A large specimen of Lineus socialis, which lives gregariously under stones on the American Atlantic coast (or of Lineus vegetus on the west coast), may fragment into six to twenty or more pieces. After transforming into complete worms of smaller size, these grow again and later reproduce sexually. Most though not all ribbon worms are of separate sexes. The eggs are usually laid in gelatinous strings or masses, and the young hatch as juvenile worms. In some species of Lineus, in Cerebratulus, and in some of their relatives, the egg hatches as a gelatinous, helmet-shaped, free-swimming little larva, called a pilidium. It must feed on microscopic organisms and develop further before it takes on the structure of the adult.

For the most part, ribbon worms are bottom dwellers on temperate marine shores, where they burrow in mud or sand or creep about among rocks and seaweeds between tide marks or in shallow waters. Only a few burrow into the deep-sea bottom, sometimes at depths of forty-five hundred feet or more. Of some 570 described species, nearly 200 are found along the Atlantic or Mediterranean shores of Europe. About 100 live on the Pacific coast of North America, at least 18 of them identical or very similar to European species. The Atlantic coast of North America has few more than 50 known species, and W. R. Coe, the American authority on nemerteans, thought this was due to the cold arctic current that comes close to the coast as far south as Cape Hatteras, for many of the missing genera are warm-temperate forms. Almost 30 species are described from Japanese shores. In the open seas, chiefly the southern parts of the North Atlantic, there are nearly 60 gelatinous species that drift or swim slowly far below the surface. They have been brought up from depths ranging from six hundred to nine thousand feet, most from below three thousand feet. Nemerteans are less common in tropical or subtropical seas, but well rep- resented in arctic and antarctic waters, often by the familiar temperate genera: Lineus, Amphiporus, Cerebratulus, and Tetrastemma.

Perhaps the most cosmopolitan species is Lineus ruber, found from Siberia to South Africa. The slender, rounded body is 3 to 9 inches long; and different varieties are colored red, green, or brown, any of them difficult to see in natural surroundings, even when one has lifted the stone under which the worm lives. Fresh waters, especially in northern latitudes, harbor species of the genus Prostoma. What seems to be a single species, Prostoma rubrum, a slender reddish worm less than 1 inch long, can be found in pools and quiet streams in nearly all parts of the United States. It clings to the leaves of aquatic plants and feeds on minute crustaceans, nematodes, and turbellarians. In Europe this genus has also an eyeless variant that lives in caves.

Land nemerteans are all of the genus Geonemertes. The two best-known species are slender, pale in color, and not more than 2 inches long. By exploiting the nemertean talent for copious secretion of slime, land nemerteans manage to live along marine shores, in moist earth, or under foliage and fallen logs, in such places as Bermuda, Australia, New Zealand, and many South Pacific islands. In the Seychelles, Geonemertes arboricola occupies the leaf bases of a screw pine (Pandanus) tree, often living high in the tree.

Only Carcinonemertes has been classed as a parasite. It lives on the gills of various crabs when it is young, and then moves to the egg masses, both feeding on the eggs and living as a commensal by eating any small animals it can find as it clings to its host. Adults of Carcinonemertes carcinophila are about 1 inch long and orange- or brick-red.

Commensal nemerteans live mostly in tunicates, sponges, or bivalves, sharing the food in the host's feeding currents. Common in the mantle cavity of various clams on European and both American coasts is Malacobdella grossa, a short, white, thick worm, with an adhesive disk at the rear. It creeps in leechlike fashion. The genus to which it belongs constitutes a separate order of nemerteans.

The other three orders contain all the more typical elongated worms; they are distinguished from each other mostly by internal characters, such as the arrangement of the muscle layers. The paleonemerteans, with an unarmed proboscis, include such forms as Tubulanus. Also with unarmed proboscis are the heteronemerteans, among them Lineus and Cerebratulus. The latter is a very large, firm, and flattened worm which lives in burrows in sand or mud and swims actively through the water. The hoplonemerteans, with an armed proboscis, are divided into two suborders. In one the members have at the tip of the proboscis a single stylet, a straight or curved thorn which pierces and holds prey. These include many quite common shore forms such as Amphiporus; the very slender Emplectonema,found among mussels and barnacles on pilings; and Paranemertes peregrina of the American west coast, often a rich purple on the upper surface. The parasitic or commensal Carcinonemertes belongs here, as do various commensal species, the fresh-water forms, and also the land nemerteans. In the second suborder, members have on the tip of the proboscis not one large stylet but a large number of minute ones. These worms include some shore species, but most float or swim in the open sea far below the surface. Many are broad, flattened worms, of yellow, orange, pink, or red hues. The drifting types are quite gelatinous, the swimming ones equipped with tail and sometimes also with side fins.

bibliographic citation
Buchsbaum, R. M. and L. J. Milne. 1960. The Lower Animals. Living Invertebrates of the World. Doubleday, Garden City, N.Y.
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Katja Schulz (Katja)
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14 Fun Facts about Marine Ribbon Worms

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Ribbon worms swallow prey whole, grease themselves with their mucus to slide quickly through mud, split into new worms if severed, and much more.
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Brief Summary

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The Nemertea, also called ribbon worms or proboscis worms, (and sometimes referred to as Rhynchocoela or Nemertini,) are a distinctive group of 1150 known species of mostly marine invertebrates, found world-wide (Gibson, 1995).

Nemerteans have an unsegmented body, thin and elongated with no differentiated head. Unlike flatworms, which they resemble in many ways, they have a separate mouth and anus, and a digestive tract that runs the full length of their body. Very distinctive is their eversible proboscis, which is stored in the body-length long fluid-filled rhyncocoel when not in use. When activated, it is forced inside out with hydrostatic pressure from body muscle contractions to wrap around its prey , often administering venom through rhabdites (holes). Some species (members of the order Hoplonemertea) are armed with a pointed stylet at the tip of the proboscis, which the animal uses to puncture and kill its prey. Since stylets are frequently lost or broken in hunting, and growing worms require larger stylets, the stylets are continually formed (in large epithelial cells) and stored so there is always one on hand to replace an old or lost one.

Nemerteans have more developed muscle than flatworms, and can contract their body to up to a tenth of their extended length. They use body muscles to locomote (and have cilia covering their epidermis and lots of gland cells for mucus production so they glide across surfaces). They also use body musculature to move food through their gut and blood through their circulatory system.

(Kozloff 1990; Brusca and Brusca 2003)

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Habitat

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While most nemerteans have a benthic marine lifestyle, they are also found in fresh water and brackish waters, and there are several terrestrial species.

(Kozloff 1990; Brusca and Brusca 2003)

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Life Cycle

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Nemerteans have simple reproduction, with many testes or ovaries built into the body wall so eggs and sperm are released directly to the outside via pores, or by breakages in the body wall. Most marine species have separate sexes, but freshwater and terrestrial species are often hermaphroditic. Usually fertilization occurs externally, with spawning induced by mating behaviors and pheremones. Mating balls can be seen containing many individuals responding to chemical cues and often coordinated spawning. Eggs develop either individually or in clumps protected in egg masses, or in a few species carried by the females in the ovaries until they hatch. In most nemertean classes development is direct, yielding oval shaped, cilia covered juveniles, which may have a pelagic lifestyle temporarily before settling into a more benthic one. There is some diversity in developmental strategies among the order Heteronemertea. Many of these species develop indirectly through a feeding, swimming larval stage called the pilidium, which looks like a swimming helmet with ciliated lobes. After swimming and feeding freely, the larval ectoderm separates to form a protective skin housing the metamorphosed juvenile inside, which lives planktonically before shedding the skin and settling on the benthos. A few species, including Micrura akkeshiensis goes through the Iwaka larval stage (similar to a pilidium larva but without the lobes). Those heteronemerteans that hatch from a benthic egg case undergo development through the Desor larval stage, which although usually classified as direct, does go through a metamorphosis. Some nemerteans can also reproduce asexually by splitting, but this is not usually a regular reproductive strategy. Many species can regenerate to various degrees, including regeneration of the proboscis.

(Kozloff 2003; Brusca and Brusca 1990)

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Size

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Most nemerteans are between 5mm and 20 cms, but one Norwegian species, Lineus longissimus gets to a length of 30 meters, and classic literature describes a report of a nemertean 60 meters long – longer than Blue whales, which are considered the world’s longest animal species (Brusca and Brusca 2003)

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Systematics and Taxonomy

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Nermertea have traditionally been considered acoelomates, and as the sister taxon to the acoelomate flatworms, and have solid bodies (Brusca and Brusca 2003). However, new morphological and molecular analyses now suggest that their body cavities, the rhynchoceol and circulatory system, may in fact be homologous to the coelomic cavities of animals within the Lophotrochozoa Halanych 2004, Tuberville et al 1992). The relationships within the phylum have also been investigated with molecular markers, and in most cases support traditional classifications (Thollesson and Norenburg 2003). There is no fossil record for the Nemerteans. This is not suprising since they have soft bodies, but even their mineralized stylets have not been found (Waggoner and Collins 2001).

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Trophic Strategy

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Mostly ferocious predators or scavengers, nemerteans hunt their prey using their eversible proboscis, a specialized organ unique to the group. While far less common, there are also filter-feedering nemerteans (that form associations with clams, tunicates and muscles, feeding within the mantles), and some that are parasites on crab species. (Brusca and Brusca 2003)

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Etymology

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Greek: "Νεμερτής " - Nemertes, sea nymph, a daughter of Nereus and Doris.

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General Description

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Nemerteans are unsegmented, dorsoventrally flattened predatory marine worms occurring at all ocean depths, freshwater, and terrestrial habitats. Approximately 1300 valid described species of the phylum Nemertea, or ribbonworms, are known worldwide. Current fieldwork suggests that at least several times this number remain to be named or discovered. Nemerteans are unsegmented worms characterized by a unique and remarkable eversible proboscis. Some are very colorful, while others are drab. They range from one millimeter to more than 30 meters long. They can be voracious predators, some are highly specialized while others are more eclectic with diets that favor other worms, crustaceans, and molluscs. They are poorly known to non-specialists because most nemerteans live in concealment, are difficult to collect, and because traditional taxonomy focuses significantly on internal anatomy based on histological study. However, many are common, abundant, and can be key predators, while the phylum itself is important to understanding evolution of early invertebrate body plans.

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Habitat

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Nemerteans hide in rocky crevices, beneath stones, algal holdfasts, or burrow into substrate.

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Known Prey Organisms

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Annelid worms, crustaceans

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Life History

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Direct or indirect development with a pilidium larva.

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Molecular Biology

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16S rDNA and cytochrome oxidase (CO1)

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Nemertea

Nemertea

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Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms.[2] Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela.[1] Most 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 anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts (turns inside-out) to emerge just above the mouth and capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.

The brain is a ring of four ganglia, positioned around the rhynchocoel near its front end. At least a pair of ventral nerve cords connect to the brain and run along the length of the body. Most nemerteans have various chemoreceptors, and on their heads some species have a number of pigment-cup ocelli. These ocelli can detect light but can not form an image. Nemerteans respire through the skin. They have at least two lateral vessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid. There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in the front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside.

All nemerteans move slowly, using their external cilia to glide on surfaces on a trail of slime, while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations. A few live in the open ocean while the rest find or make hiding places on the bottom. About a dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places. Most nemerteans are carnivores, feeding on annelids, clams and crustaceans. A few species are scavengers, and a few species live commensally inside the mantle cavity of molluscs. Some species have devastated commercial fishing of clams and crabs. Nemerteans have few predators. Two species are sold as fish bait.

In most species the sexes are separate, but all the freshwater species are hermaphroditic. Nemerteans often have numerous temporary gonads (ovaries or testes), and build temporary gonoducts (ducts from which the ova or sperm are emitted), one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. The fertilized egg divides by spiral cleavage and grows by determinate development, in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or to form planuliform larvae, in which the larva's long axis is the same as the juvenile's. However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some species fragment readily, and even parts near the tail can grow full bodies.

It has been suggested that three fossil species may be nemerteans, but none is confirmed. Traditional taxonomy divides the phylum in two classes, Anopla ("unarmed" – their proboscises do not have a little dagger) with two orders, and Enopla ("armed" with a dagger) with two orders. However, it is now accepted that Anopla are paraphyletic (have given rise to another group), as one order is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea is monophyletic. Its synapomorphies include the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related to flatworms. Both phyla are regarded as members of the Lophotrochozoa, a very large "super-phylum" that also includes molluscs, annelids, brachiopods, bryozoa and many other protostomes.

History

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.[3] In 1995, a total of 1,149 species had been described and grouped into 250 genera.[4]

Nemertea are named after the Greek sea-nymph Nemertes, one of the daughters of Nereus and Doris.[5] Alternative names for the phylum have included Nemertini, Nemertinea, and Rhynchocoela.[1] The Nemertodermatida are a separate phylum, whose closest relatives appear to be the Acoela.[6][7]

Description

Body structure and major cavities

Nemerteans are very unusual animals.[8]

The typical nemertean body is very slim 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][Note 1] although L. longissimus is usually only a few millimeters wide.[8] The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species,[8][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][17] and some of these are much less stretchy.[8] 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 cuticle but consists of a ciliated and glandular epithelium containing rhabdites,[10] which form the mucus in which the cilia glide.[18] Each ciliated cell has many cilia and microvilli.[9] The outermost layer rests on a thickened basement membrane, the dermis.[10] Next to the dermis are at least three layers of muscles, some circular and some longitudinal.[9] The combinations of muscle types vary between the different classes, but these are not associated with differences in movement.[10] Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species.[9] Inside the concentric tubes of these layers is mesenchyme, a kind of connective tissue.[10] In pelagic species this tissue is gelatinous and buoyant.[9]

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 anus is at the tip of the tail.[19] Above the gut and separated from the gut by mesenchyme is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class Anopla has an orifice a little to the front of the mouth, but still under the front of the body. In the other class, Enopla, the mouth and the front of the rhynchocoel share an orifice.[9] The rhynchocoel is a coelom, as it is lined by epithelium.[10]

Proboscis and feeding

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 in the rhynchocoel, the pressure makes the proboscis jump inside-out to attack the animal's prey 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, and which can stretch up to 30 times its inactive length and then retract the proboscis.[9]

"
Gorgonorhynchus repens, a species within class Anopla, discharges a sticky branched proboscis.

The proboscis of the class Anopla ("unarmed"[20]) exits from an orifice which is separate from the mouth,[9] coils around the prey and immobilizes it by sticky, toxic secretions.[19] The Anopla can attack as soon as they move into the range of the proboscis.[21] 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]

"
Stylet-containing part of proboscis of "armed" nemertean Amphiporus ochraceus.

In most of the class Enopla ("armed"[20]), 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.[19] The stylet is attached about one-third of 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.[20][22] The Enopla can only attack after contacting the prey.[21]

Some nemerteans, such as L. longissimus, absorb organic food in solution through their skins, which may make the long, slim bodies an advantage.[8] Suspension feeding is found only among the specialized symbiotic bdellonemerteans,[21] which have a proboscis but no stylet, and use suckers to attach themselves to bivalves.[23]

Respiration and circulatory system

Nemerteans lack specialized gills, and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid circulation rather than diffusion to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels,[24] while their blood is contained in the main body cavity.[25] The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluid skeleton in everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation.[24] The circulatory vessels are a system of coeloms.[26]

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,[27] 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.[24] 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.[24]

Excretion

"
A schematic representation of a flame cell and other associated structures

Nemertea use organs called protonephridia[24] to excrete soluble waste products, especially nitrogenous by-products of cellular metabolism.[28] In nemertean protonephridia, flame cells which filter out the wastes are embedded in the front part of the two lateral fluid vessels. The flame cells remove the wastes into two collecting ducts, one on either side, and each duct has one or more nephridiopores through which the wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be that osmoregulation is more difficult in non-marine environments.[24]

Nervous system and senses

"
Brain and neural cords of hoplonemertean Amphiporus ochraceus. Several clusters of dark eyespots and the opening of one cerebral organ are also visible.

The central nervous system consists of a brain and paired ventral nerve cords that connect to the brain and run along the length of the body. The brain is a ring of four ganglia, masses of nerve cells, positioned round the rhynchocoel near its front end[29] – while the brains of most protostome invertebrates encircle the foregut.[30] Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord.[29] In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers.[31] The central nervous system is often red or pink because it contains hemoglobin. This stores oxygen for peak activity or when the animal experiences anoxia, for example while burrowing in oxygen-free sediments.[29]

Some species have paired cerebral organs, sacs whose only openings are to the outside. Others species have unpaired evertible organs on the front of their heads. Some have slits along the side of the head or grooves obliquely across the head, and these may be associated with paired cerebral organs. All of these are thought to be chemoreceptors, and the cerebral organs may also aiding osmoregulation. Small pits in the epidermis appear to be sensors.[29] On their head some species have a number of pigment-cup ocelli,[29] which can detect light but not form an image.[32] Most nemerteans have two to six ocelli, although some have hundreds.[31] A few tiny species that live between grains of sand have statocysts,[29] which sense balance.[33]

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.[19]

Movement

The nemertean Geonemertes pelaensis (right) being inspected by a spider, which it then captures.
"
Lineus longissimus in Grevelingen

Nemerteans generally move slowly,[10] though they have occasionally been documented to successfully prey on spiders or insects.[34] 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 towards the object.[20]

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.[35] All reproduce sexually, and most species are gonochoric (the sexes are separate),[10][35] but all the freshwater forms are hermaphroditic.[25]

Nemerteans often have numerous temporary gonads (ovaries or testes), forming a row down each side of the body in the mesenchyme.[25][35] Temporary gonoducts (ducts from which the ova or sperm are emitted[36]), one per gonad, are built when the ova and sperm are ready.[35] The eggs are generally fertilised externally. Some species shed them into the water, some lay them in a burrow or tube, and some protect them by cocoons or gelatinous strings.[35] Some bathypelagic (deep sea) species have internal fertilization, and some of these are viviparous, growing their embryos in the female's body.[25][35]

The zygote (fertilised egg) divides by spiral cleavage and grows by determinate development,[35] in which the fate of a cell can usually be predicted from its predecessors in the process of division.[8] The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or to form planuliform larvae,[35] in which the larva's long axis is the same as the juvenile's.[30] The planuliform larva stage may be short-lived and lecithotrophic ("yolky") before becoming a juvenile,[35] or may be planktotrophic, swimming for some time and eating prey larger than microscopic particles.[30] However, many members of the order Heteronemertea and the palaeonemertean family Hubrechtidae form a pilidium larva, which can capture unicellular algae and which Maslakova describes as like a deerstalker cap with the ear flaps pulled down. It has a gut which lies across the body, a mouth between the "ear flaps", but no anus. A small number of imaginal discs form, encircling the archenteron (developing gut) and coalesce to form the juvenile. When it is fully formed, the juvenile bursts out of the larva body and usually eats it during this catastrophic metamorphosis.[30] This larval stage is unique in that there are no Hox genes involved during development, which are only found in the juveniles developing inside the larvae.[37]

The species Paranemertes peregrina has been reported as having a life span of around 18 months.[31]

Ecological significance

"
A terrestrial nemertean from West Java. The animal is 1.5 centimetres (0.59 in) long, of which the anterior 1 centimetre (0.39 in) is visible.
"
A terrestrial Geonemertes sp. on a rotting log, from Mindanao Island, the Philippines

Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or the holdfasts of algae or sessile animals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined with mucus or produce cellophane-like tubes. Mainly in the tropics and subtropics, about 12 species appear in freshwater,[9] and about a dozen species live on land in cool, damp places, for example under rotting logs.[8]

The terrestrial Argonemertes dendyi is a native of Australia but has been found in the British Isles, in Sao Miguel in the Azores, in Gran Canaria, and in a lava tube at Kaumana on the Island of Hawaii. It can build a cocoon, which allows it to avoid desiccation while being transported, and it may be able to build populations quickly in new areas as it is a protandrous hermaphrodite.[38] Another terrestrial genus, Geonemertes, is mostly found in Australasia but has species in the Seychelles, widely across the Indo-Pacific, in Tristan da Cunha in the South Atlantic, in Frankfurt, in the Canary Islands, in Madeira and in the Azores.[4] Geonemertes pelaensis has been implicated in the decline of native arthropod species on the Ogasawara Islands, where it was introduced in the 1980s.[39]

Most are carnivores, feeding on annelids, clams and crustaceans,[19] and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects and myriapods are the only known prey of the two terrestrial species of Argonemertes.[21] A few nemerteans are scavengers,[19] and these generally have good distance chemoreception ("smell") and are not selective about their prey.[21] A few species live commensally inside the mantle cavity of molluscs and feed on micro-organisms filtered out by the host.[40]

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.[21] 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.[25] The American Cerebratulus lacteus and the South African Polybrachiorhynchus dayi, both called "tapeworms" in their respective localities, are sold as fish bait.[9]

Taxonomy

  • Class Anopla ("unarmed"). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.[20]
    • Order Palaeonemertea. Comprises 100 marine species. Their body wall has outer circular and inner length-wise muscles. In addition, Carinoma tremaphoros has circular and inner length-wise muscles in the epidermis; the extra muscle layers seem to be needed for burrowing by peristalsis.[20]
    • 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.[20]
  • Class Enopla ("armed"). All have stylets except order Bdellonemertea. Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.[20]
    • Order Bdellonemertea. Includes seven species, of which six live as commensals in the mantle of large clams and one in that of a freshwater snail. The hosts 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.[20]
    • Order Hoplonemertea. Comprises 650 species. They live in benthic and pelagic sea water, in freshwater and on land. They feed by commensalism and parasitism, and are armed with stylet(s)[20]
      • Suborder Monostilifera. Includes 500 species with a single central stylet. Some use the stylet for locomotion as well as for capturing prey.[20]
      • Suborder Polystilifera. Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets.[20]

Evolutionary history

Fossil record

As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare. [10][40] Knaust (2010) reported nemertean fossils and traces from the Middle Triassic of Germany.[41] 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][40]

The Middle Cambrian fossil Amiskwia from the Burgess Shale has been classed as a nemertean, based on a resemblance to some unusual deep-sea swimming nemerteans, but few paleontologists accept this classification as the Burgess Shale fossils show no evidence of rhynchocoel nor intestinal caeca.[40][42]

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.[43]

It has been suggested that Archisymplectes, one of the Pennsylvanian-age animals from Mazon Creek in northern and central Illinois, may be a nemertean.[44] This fossil, however, only preserves the outline of the "worm",[40] and there is no evidence of a proboscis,[45] so there is no certainty that it represents a nemertean.[40]

Within Nemertea

Nemertea  

Palaeonemertea

     

Heteronemertea

Enopla

Bdellonemertea

Hoplonemertea

Monostilifera

   

Polystilifera

         
Groups within Nemertea, by Ruppert, Fox and Barnes (2004).[46]
highlights the "Anopla", which are paraphyletic.[46]

There is no doubt that the phylum Nemertea is monophyletic (meaning that the phylum includes all and only descendants of one ancestor that was also a member of the phylum). The synapomorphies (trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel.[47]

While Ruppert, Fox and Barnes (2004) treat the Palaeonemertea as monophyletic,[46] Thollesson and Norenburg (2003) regard them as paraphyletic and basal (contains the ancestors of the more recent clades).[47] 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.[46][47] Ruppert, Fox and Barnes (2004) treat the Bdellonemertea as a clade separate from the Hoplonemertea,[46] 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.[47] The Polystilifera (with many tiny stylets) are monophyletic.[46][47]

Relationships with other phyla

English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre-cladistics analyses emphasised as shared features: multiciliated (with multiple cilia per cell), glandular epidermis; rod-shaped secretory bodies or rhabdites; frontal glands or organs; protonephridia; and acoelomate body organization.[48] However, multiciliated epidermal cells and epidermal gland cells are also found in Ctenophora, Echiura, Sipuncula, Annelida, Mollusca and other taxa. The rhabdites of nemertea have a different structure from those of flatworms at the microscopic scale. The frontal glands or organs of flatworms vary a lot in structure, and similar structures appear in small marine annelids and entoproct larvae. The protonephridia of nemertea and flatworms are different in structure,[48] 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,[24] while the flame cells of flatworms are scattered throughout the body.[49] Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia.[48]

According to more recent analyses, in the development of nemertean embryos, ectomesoderm (outer part of the mesoderm, which is the layer in which most of the internal organs are built) is derived from cells labelled 3a and 3b, and endomesoderm (inner part of the mesoderm) is derived from the 4d cell. Some of the ectomesoderm in annelids, echiurans and molluscs is derived from cells 3a and 3b, while the ectomesoderm of polyclad flatworms is derived from the 2b cell and acoel flatworms produce no ectomesoderm. In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellular connective tissue. This structure is similar to that found in larger flatworms such as polyclads and 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.[48]

Bilateria

Acoelomorpha (Acoela and Nemertodermatida)

     

Deuterostomia (Echinoderms, chordates, etc.)

Protostomia

Ecdysozoa
(Arthropods, nematodes, priapulids, etc.)

Lophotrochozoa

Bryozoa

     

Annelida and phyla merged into them

   

Mollusca

   

Phoronida and Brachiopoda

   

Nemertea

   

Dicyemida

   

Myzostomida

Platyzoa    

Platyhelminthes

   

Gastrotricha

     

Other Platyzoa

             
Relationships of Nemertea to other Bilateria:[50][Note 2]

Nemerteans' affinities with Annelida (including Echiura, Pogonophora, Vestimentifera and perhaps Sipuncula) and Mollusca make the ribbon-worms members of Lophotrochozoa, which include about half of the extant animal phyla.[52] Lophotrochozoa groups: those animals that feed using a lophophore (Brachiopoda, Bryozoa, Phoronida, Entoprocta); phyla in which most members' embryos develop into trochophore larvae (for example Annelida and Mollusca); and some other phyla (such as Platyhelminthes, Sipuncula, Gastrotricha, Gnathostomulida, Micrognathozoa, Nemertea, Phoronida, Platyhelminthes and Rotifera).[50][52] These groupings are based on 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.[50][52]

Most protostome phyla outside the Lophotrochozoa are members of Ecdysozoa ("animals that molt"), which include Arthropoda, Nematoda and Priapulida. Most other bilaterian phyla are in the Deuterostomia, which include Echinodermata and Chordata. The Acoelomorpha, which are neither protostomes nor deuterostomes, are regarded as basal bilaterians.[50][52][53]

Notes

  1. ^ 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:
  2. ^ Sipuncula were merged into Annelida in 2007.[51]

See also

References

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Nemertea: Brief Summary

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Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms. Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Most 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 anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts (turns inside-out) to emerge just above the mouth and capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.

The brain is a ring of four ganglia, positioned around the rhynchocoel near its front end. At least a pair of ventral nerve cords connect to the brain and run along the length of the body. Most nemerteans have various chemoreceptors, and on their heads some species have a number of pigment-cup ocelli. These ocelli can detect light but can not form an image. Nemerteans respire through the skin. They have at least two lateral vessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid. There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in the front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside.

All nemerteans move slowly, using their external cilia to glide on surfaces on a trail of slime, while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations. A few live in the open ocean while the rest find or make hiding places on the bottom. About a dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places. Most nemerteans are carnivores, feeding on annelids, clams and crustaceans. A few species are scavengers, and a few species live commensally inside the mantle cavity of molluscs. Some species have devastated commercial fishing of clams and crabs. Nemerteans have few predators. Two species are sold as fish bait.

In most species the sexes are separate, but all the freshwater species are hermaphroditic. Nemerteans often have numerous temporary gonads (ovaries or testes), and build temporary gonoducts (ducts from which the ova or sperm are emitted), one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. The fertilized egg divides by spiral cleavage and grows by determinate development, in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or to form planuliform larvae, in which the larva's long axis is the same as the juvenile's. However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some species fragment readily, and even parts near the tail can grow full bodies.

It has been suggested that three fossil species may be nemerteans, but none is confirmed. Traditional taxonomy divides the phylum in two classes, Anopla ("unarmed" – their proboscises do not have a little dagger) with two orders, and Enopla ("armed" with a dagger) with two orders. However, it is now accepted that Anopla are paraphyletic (have given rise to another group), as one order is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea is monophyletic. Its synapomorphies include the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related to flatworms. Both phyla are regarded as members of the Lophotrochozoa, a very large "super-phylum" that also includes molluscs, annelids, brachiopods, bryozoa and many other protostomes.

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