Nemertea

ribbon worms

(Also: proboscis worms)

Features
By Jeremy Wright

Diversity

Phylum Nemertea , or ribbon worms, contains approximately 1,200 species of bilaterally symmetrical, coelomate, unsegmented worms, which are divided into two classes, each of which is divided into two orders ( Anopla : Palaeonemertea , Heteronemertea and Enopla : Hoplonemertea , Bdellonemertea ). Distinctions between the classes and orders lie in the presence of body armoring ( Anopla has no armor and Enopla is typically armed with stylets), proboscis construction, mouth position relative to the cerebral ganglion, and body morphology ( Enopla is morphologically specialized into three regions (with the exception of Bdellonemerta ), while Anopla is not), as well as gut shape, body wall muscle layering, and positioning of longitudinal nerve cords. They have widely varied distributions and habitats; most species are free-swimming, benthic, marine organisms, but approximately 100 species are known from deep-sea environments, and planktonic, symbiotic, freshwater and even terrestrial species exist. They typically prey on small invertebrates and their eggs, using a uniquely structured proboscis, but some species are known to feed on plant material and, in some commensal species, phytoplankton captured by their hosts. Nemertean species also exhibit a wide range in size and length, from a few millimeters to several meters (stretched lengths may reach over 30 meters, however), and may be drab or very brightly colored.

Geographic Range

Nemerteans are found world-wide. The majority of species in this phylum are marine, benthic, or littoral animals, known from intertidal to deep sea environments. A few are terrestrial or ectoparasitic (mainly species in suborder Monostilifera ), and around 20 species are found in freshwater environments.

Habitat

Typically marine, benthic animals, nemerteans may be found burrowed in mud, sand, or other sediments, among rocks, or associated with algae or other plant masses. Freshwater nemerteans are found in similar habitats in streams and pools. Commensal or parasitic species may be found on crustaceans or in tunicates , sponges or bivalves , while terrestrial species most often live along shorelines, in moist soil.

Systematic and Taxonomic History

Nemerteans have a long and complicated taxonomic history, with the first named species, Lineus longissimus (originally named Ascaris longissima ) being described by William Borlase in 1758, and even earlier by the Swedish naturalist Olaus Magnus in 1555. For much of their early history, nemerteans were included in genus Planaria (phylum Platyhelminthes ), until, in 1817, the French naturalist Georges Cuvier recognized their differences and, unaware of the previous names applied to the species, placed L. longissimus in the genus Nemertes , from which the phylum would eventually derive its name. They were still, however, thought to be closely related to planarians and other turbellarian species, undergoing a number of different changes in name and internal classification over the following hundred years or so, until finally being separated from flatworms by Charles Minot in 1876. Nemerteans would not be fully recognized and accepted as a valid phylum, however, until the mid-20th century.

While phylum Nemertea is accepted as monophyletic, the evolutionary relationships within the phylum have been the subject of some debate, as is the monophyly of several orders. Recent molecular evidence has the monogeneric order Bdellonemertea nested within, and subsumed by the monophyletic order Hoplonemertea (which contains the two monophyletic suborders Monostilifera and Polystillifera ). The order Paleonemertea , as traditionally understood, appears to be polyphyletic, due to a sister relationship between Hubrechtella dubia and the order Heteronemertea (which appears to be monophyletic). The remaining paleonemerteans do form a monophyletic group, however, and appear to be the sister group to the other two nemertean orders and family Hubrechtidae . This phylogenetic arrangement also renders the traditional nemertean classes Anopla and Enopla nonmonophyletic, which will necessitate future, larger-scale changes to the taxonomic classifications within this group.

Morphological phylogenetic analyses have considered nemerteans to be most closely related with flatworms (phylum Platyhelminthes ). Recent molecular analyses, however, suggest Nemertea belongs to superphylum Lophotrochozoa , as do annelids , molluscs , and several other smaller phyla. Their relationship to other groups within this superphylum remains unresolved, although recent, phylogenomic data suggests they constitute the sister group to mollusks, within a clade named Eutrochozoa , which also includes annelid worms.

Physical Description

Nemerteans range in length from a few millimeters to 30 meters (stretched) in length (most commonly 20 cm or less) and may be drab or very brightly colored. They are unsegmented, without a differentiated head, and are most often flattened (occasionally convex dorsally or cylindrical). They have very thickly muscled body walls covered by a mesenchyme, and a ciliated epidermis comprised of columnar epithelial cells, as well as some sensory and mucous gland cells. The thickness of the epidermis and mesenchyme varies from species to species. Organization of the muscle wall is also variable, but it is typically made up of longitudinal and circular muscles in two or three layers. The interior of the body wall is coated again with mesenchyme, which includes a gel matrix, and sometimes dorsoventrally oriented musculature. The muscles of the body wall provide support to the worm and enable it to alter its length and width dramatically and quickly, changes which are often used for movement. While considered "functionally acoelomate organisms," with solid bodies, the nemertean rhynchoceol and some blood vessels are true acoelomic cavities.

Ribbon worms have a simple circulatory system consisting of vessels and thin walled lacunae. Although this system can vary in complexity depending on species, the most basic arrangement is a pair of longitudinal vessels that extend the entire length of the body and connect to cephalic and anal lacunae. The blood is a colorless fluid in which cells such as corpuscles (some of which may contain hemoglobin), lymphocytes and leukocytes can be found; the blood does not circulate in any particular direction. This system is mainly involved with transporting nutrients, gases, secretions and wastes through the animal’s body and may aid in regulating hydrostatic pressure. Oxygen and carbon dioxide diffuse across the animal’s body surface.

Nemerteans have a central nervous system consisting of a complex cerebral ganglion with four connected lobes, giving rise to a pair of longitudinal, gangliated nerve cords from the ventral lobes. The nerve cords connect to each other at points throughout the worm’s body and give rise to peripheral sensory and motor nerves.

The proboscis apparatus of these animals is unique to the phylum. The proboscis is an elongate, eversible, blind tube, surrounded by the rhynchocoel (a fluid filled, hydrostatic cavity) and additional muscle layers, including the proboscis retractor muscle at the posterior wall of the rhynchocoel. It is either connected directly to the foregut or opens through a proboscis pore. In many species, the proboscis bears stylets, structures typically 50 to 200 µm in length, each shaped like a nail. Nemerteans may use their proboscises for locomotion, though its primary function is in prey capture. Stylets are often lost during prey capture and must be replaced as the animal grows; they are continuously formed by specialized cells (stelotocytes) within reverse stylet sacs, where they are stored. Sticky, sometimes even toxic substances are produced by the proboscis in order to trap and immobilize prey.

Sexual dimorphism is rare but not unknown in nemerteans. Some sexually dimorphic characteristics include body size (females larger), differences in gonad location, along with sexual appendages in males and coloration differences during breeding season (males tend to be brighter in color).

  • Sexual Dimorphism
  • sexes alike
  • female larger
  • male more colorful
  • sexes shaped differently
  • ornamentation

Development

Nemertean fertilization may be external or internal. Egg cases or capsules are formed, in which development occurs. Some terrestrial, freshwater and deep sea nemerteans are ovoviviparous. Embryonic development varies between species. Cleavage is holoblastic and spiral. Some species undergo gradual direct development; those who do, may develop within egg cases, nourished by a yolk until they hatch, undergoing no abrupt metamorphosis. Others produce a larval form known as a Desor larva, which undergoes metamorphosis while still in its egg capsule (this is considered a form of indirect development by some researchers). Other nemertean species undergo different types of indirect development, beginning life as pilidium (free swimming larvae). Pilidium either develop into juveniles and adults within a protective larval skin, or as Iwata larvae, undergoing lechithothrophic development, also within a larval skin. Asexual reproduction is also known, via transverse fission into small fragments, which form mucous cysts, in which regeneration occurs. There are also protandric and simultaneous hermaphroditic species (most often in freshwater species).

Reproduction

The gonads of sexually reproductive nemerterans are specialized patches of mesenchymal tissue arranged along each side of the intestine, alternating with the intestinal diverticula. Prior to breeding, the gonads enlarge and become hollow, while specialized cells in the testes and ovaries generate sperm and eggs into the newly created lumina, filling the area between the gut and body wall. The worms become increasingly active once they are almost ready to mate and initiate mating behavior. Spawning is triggered by tactile and chemical cues. Worms join together in a mating mass, releasing gametes through temporary pores or body wall ruptures, and fertilization is typically external, sometimes in a mass of mucus. Some species exhibit internal fertilization: released sperm move through mucus surrounding the mating worms, entering females’ ovaries. There is evidence that at least one species ( Lineus viridis ) is polyandric; multiple males are found moving within a mating cocoon formed by one female.

Most nemerteans are dioecious, although both sequential and protandric hermaphrodites are known, as is asexual reproduction via transverse fission. Fertilization may be external or internal. There is evidence that sexual maturity is reached after the release of neurosecretory hormones, possibly released by the cerebral organ complex. Gestational period and age at sexual maturity have not been recorded for members of this phylum.

There is no parental investment by nemerteans beyond the production of gametes and, in the case of those species which are ovoviparous, protection until birth.

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • protecting
      • female

Lifespan/Longevity

Although life span has not been widely studied in nemerteans, at least one species ( Paranemertes peregrina ) has a reported life span of about 18 months.

Behavior

Some smaller nemerteans move using their epithelial cilia while many species, particularly terrestrial ones, use mucus to create a smooth, gliding surface; some larger animals swim or drift through the water. Outside of breeding, when they can be found in breeding "knots," these worms tend to be solitary.

Communication and Perception

Nemerteans are highly sensitive to touch, due to ciliated epidermal cells scattered over their surfaces. They also have anywhere from two to hundreds of anterior eyes, usually pigment-cup occeli, although a few species have lensed eyes that are highly sensitive to light intensity and direction. There are a few European cave-dwelling species that appear to have secondarily lost eyes.

These animals are also highly chemosensitive and it is likely that structures including cephalic slits, cerebral organs, and frontal glands act as chemoreceptors. When present, pores located in the cephalic slits lead to a ciliated cerebral canal, the inner ends of which are surrounded by nervous and glandular tissue. Glands associated with the frontal sense organ receive nerves from the cerebral ganglion and may also aid in chemoreception. These animals use chemoreception to locate food, mates, and to analyze the substrate and water around them.

Food Habits

Nemerteans are hunters and scavengers. Hunting methods vary between species and depend on prey type. Sticky, sometimes toxic substances are produced by the proboscis in order to trap and immobilize prey. In some cases, the proboscis is everted when the worm comes into contact with prey; it then coils around the prey, which is swallowed whole. In other instances, prey may be stabbed by a stylet, with toxins delivered through the resulting wound. The prey is then either swallowed, using peristaltic action of the body wall as well as ciliary currents within the gut, or injected with digestive enzymes and consumed suctorially. Typical prey includes small invertebrates, including bivalves , polychaetes , crustaceans , insects , and their eggs; some species feed primarily on plant material and, in commensal species, on phytoplankton captured from hosts.

Once food has been swallowed, it leads to a highly ciliated foregut, comprised of a buccal cavity, often a short esophagus, and a stomach, which occasionally possesses enzymatic gland cells. The foregut also has a number of mucus producing cells or glands. The stomach leads to a long, straight midgut with many lateral diverticulae; these are lined with vacuolated, ciliated, phagocytic cells, which also bear microvilli. This greatly increases the surface area available for nutrient absorption. Food is stored in the walls of the midgut as fats (occasionally as glycogen). The animal’s circulatory system absorbs these products and undigestible matter is moved to the rectum and on to the anus, where it is excreted. Nemerteans have flame bulb protonephridia (anywhere from two to thousands), which are typically associated with blood vessels. In the most simple instances, a pair of flame bulbs are connected to two nephridioducts, each of which has a lateral nephridiopore. In more complex arrangements, flame blubs may occur in clusters or have multiple ducts and there may be thousands of pores. It is most likely that metabolic products such as nitrogenous wastes as well as salts are removed by the protonephridia; they also may play an important role in osmoregulation.

Predation

Although they are soft-bodied worms, nemerteans do not have a large number of natural predators. Sharp stylets serve to deter predators in those species that possess them. The toxic secretions of these worms are also a predator deterrent. Nemerteans are able to regenerate parts of their bodies if lost; enabling them to survive attempted predation, and some brightly colored species may be aposematic (particularly those lacking stylets). Known predators include birds , fellow nemerteans , and occasionally other invertebrates.

Ecosystem Roles

Most nemerteans are free living. However, some are parasitic, living first on the gills and later preying on the egg masses of crabs. Parasitism by Carcinonemertes species in particular has caused population declines (up to 55% egg mortality) of Dungeness crabs ( Cancer magister ) in California.

All members of the order Bdellonemertea are commensal species with a variety of tunicates , sponges , and bivalves , filter feeding from the water flow produced by their hosts.

Nemerteans may be parasitized by various protozoans.

Species Used as Host
Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Beyond scientific research, there are no known positive effects of nemerteans on humans

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of nemerteans on humans.

Conservation Status

As a cosmopolitan phylum, nemerteans do not seem to be in danger of becoming threatened or endangered.

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

native range

the area in which the animal is naturally found, the region in which it is endemic.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

native range

the area in which the animal is naturally found, the region in which it is endemic.

native range

the area in which the animal is naturally found, the region in which it is endemic.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

native range

the area in which the animal is naturally found, the region in which it is endemic.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

terrestrial

Living on the ground.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

freshwater

mainly lives in water that is not salty.

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

abyssal

on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).

brackish water

areas with salty water, usually in coastal marshes and estuaries.

marsh

marshes are wetland areas often dominated by grasses and reeds.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

bog

a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

polyandrous

Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

seasonal breeding

breeding is confined to a particular season

protandrous

condition of hermaphroditic animals (and plants) in which the male organs and their products appear before the female organs and their products

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

asexual

reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

fertilization

union of egg and spermatozoan

external fertilization

fertilization takes place outside the female's body

internal fertilization

fertilization takes place within the female's body

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

natatorial

specialized for swimming

diurnal
  1. active during the day, 2. lasting for one day.
nocturnal

active during the night

crepuscular

active at dawn and dusk

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

motile

having the capacity to move from one place to another.

solitary

lives alone

tactile

uses touch to communicate

chemical

uses smells or other chemicals to communicate

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

visual

uses sight to communicate

tactile

uses touch to communicate

chemical

uses smells or other chemicals to communicate

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

carnivore

an animal that mainly eats meat

insectivore

An animal that eats mainly insects or spiders.

molluscivore

eats mollusks, members of Phylum Mollusca

omnivore

an animal that mainly eats all kinds of things, including plants and animals

planktivore

an animal that mainly eats plankton

detritivore

an animal that mainly eats decomposed plants and/or animals

coprophage

an animal that mainly eats the dung of other animals

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Wickham, D. 1979. Predation by the Nemertean Carcinonemerts errans on eggs of the Dungeness Crab Cancer magister . Marine Biology , 55: 45-53. Accessed March 20, 2013 at http://link.springer.com/article/10.1007/BF00391716#page-1 .

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To cite this page: Wright, J. 2014. "Nemertea" (On-line), Animal Diversity Web. Accessed {%B %d, %Y} at https://animaldiversity.org/accounts/Nemertea/

Last updated: 2014-51-02 / Generated: 2025-10-03 00:57

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