Ribbon worms, also called nemerteans, are usually slim, vermiform invertebrates that constitute the phylum Nemertea. They are characterized by an eversible infolding of the body wall, the proboscis, lying in a coelomic cavity just above the gut. These animals are also referred to as proboscis worms owing to this feature and often bear patterns of yellow, orange, red, and green on their bodies.
These worms move slowly by contracting and expanding their bodies, gliding on cilia embedded in the outermost layer of the epithelium. Though most species live in marine habitats of the tropics and subtropics, about a dozen inhabit freshwater environments, whereas some live in terrestrial habitats. They are mostly carnivores, feeding on other invertebrates, like annelids, mollusks, and crustaceans; however, they sometimes scavenge on detritus or live off the flesh of their hosts.
These worms are less than 7.9 in (20 cm) long, while many exceed 3.3 ft (1 m) in length. While the smallest of ribbon worms are barely a few millimeters long, the longest to date, probably a specimen of Lineus longissimus, measures 177 ft (54 m). A few, like Malacobdella grossa, have relatively short and wide bodies, measuring around 1.4 in (3.5 cm) in length and 0.39 in (1 cm) in width.
They can sometimes stretch their bodies up to 10 times their resting length but shrink by 50% and increase their width up to 300% when attacked.
Their bodies are heavily muscled and covered by an outermost layer of ciliated and glandular epithelium, resting on a thick basement membrane called the dermis. Beneath the dermis lies at least three layers of muscles, a mix of circular and longitudinal ones. Within these muscular layers lies a connective tissue layer called the mesenchyme, which, in pelagic worms, is gelatinous and aids in buoyancy.
The mouth is located ventrally, slightly behind the front of the body. The foregut, stomach, and intestine lie slightly below their body’s midline, and the anus is located at the tip of the tail. A coelomic cavity called the rhynchocoel typically runs above the midline and the gut. It is lined by epithelium and ends a little before the rear end of the body.
The rhynchocoel contains an eversible infolding of the body wall called the proboscis. It is attached to the back of the rhynchocoel by a muscle and, under increased pressure, jumps inside-out through a canal called the rhynchodeum that opens to the exterior through an orifice, the proboscis pore.
In members of the class Anopla, the rhynchocoel has an orifice in front of the mouth, whereas in members of the class Enopla, the mouth and the orifice open together.
These worms are typically unsegmented, though pseudosegmentation (excluding the coelom and body wall) is observed in Annulonemertes minusculus.
Ribbon worms lack a heart, and circulation typically occurs through the rhynchocoel and its peripheral vessels. While the rhynchocoel has its local circulation, the rest of the boy is well connected by circulatory vessels that transport blood throughout the body. The flow of fluid is dependent on the contraction of the body wall muscles.
In its simplest form, the circulatory system consists of two lateral vessels joined at their ends; however, in more complex forms, additional long, crisscross vessels are found.
The fluid that moves through the vessels is mostly colorless but sometimes contains yellow, orange, green, or red cells. The red cells contain hemoglobin, but the composition of the other cell types is still unknown.
These worms lack gills and thus respire through their body surface.
Soluble waste is excreted through their excretory organ protonephridia. This organ contains specialized excretory cells called flame cells that expel wastes into two collecting ducts (nephridioducts, one on either side) that drain through minute pores called nephridiopores. In complex arrangements, flame cells exist in clusters having multiple nephridioducts and thousands of nephridiopores.
Semiterrestrial and freshwater species have more flame cells than their marine counterparts.
A brain and a series of ventral nerve cords characterize the central nervous system of these worms. The brain comprises a ring of four ganglia encircling the rhynchocoel near the front end. Though most ribbon worms have a single pair of nerve cords, many species possess additional paired cords or even dorsal ones.
Some ribbon worms possess paired sac-like cerebral organs on their heads, while others have unpaired evertible chemosensory organs. The cerebral organs are assumed to play a role in osmoregulation. Additionally, a series of small pits in the epidermis of these worms also serve sensory functions. Sometimes, some worms living between grains of sand also possess special balancing organs called statocysts.
Most ribbon worms have two to six simple eyes, called pigment-cup ocelli, on their heads, though some species can have hundreds. However, these eyes are absent in a few European cave-dwelling species.
They have multiple temporary gonads (both testes and ovaries), which are specialized patches of mesenchyme along each side of the intestine. When the sperm and ova are produced, these gonads form temporary gonoducts (one per gonad) for draining the gametes.
The phylum Nemertea gets its name from the Greek sea nymph Nemertes, daughter of Nereus and Doris. Some of its alternative names are Nemertini, Nemertinea, and Rhynchocoela.
Traditionally, all ribbon worms were grouped under two classes and four orders.
According to the most recent classification scheme (2021), around 1300 known ribbon worm species fall under 2 superclasses (and 1 incertae sedis), 3 classes, and 8 orders.
While Nemertea is a monophyletic group, its main synapomorphies being the rhynchocoel and the proboscis, evolutionary relationships within the phylum are constantly debated.
Though Ruppert, Fox & Barnes (2004a) consider the class Palaeonemertea monophyletic, Thollesson & Norenburg (2003) regarded them as a paraphyletic and basal group. This debate extends to the placement of the order Bdellonemertea in the classification of these worms. While Ruppert, Fox & Barnes (2004a) consider this group as a clade outside the order Hoplonemertea, Thollesson & Norenburg (2003) believed they are a part of the group Monostilifera (which, in turn, falls under Hoplonemertea).
The order Polystilifera within Hoplonemertea is monophyletic.
Before cladistic research, ribbon worms were classified as acoelomate bilaterians, closely related to flatworms (phylum Platyhelminthes). Such consideration was based on shared traits of the two groups, such as glandular epidermis, rod-shaped secretory bodies (rhabdites), frontal glands, and protonephridia. However, many of these features were common to other invertebrates, like echiurans and other annelids. Moreover, despite their similarities with flatworms, further cladistic analyses suggested multiple differences, especially in their developmental patterns.
Recent studies based on molecular phylogeny reveal these worms are most closely related to annelids and mollusks and are grouped along with them under the broader group Lophotrochozoa. However, the exact relationship between the members of this group remains uncertain.
Although these worms are soft-bodied and fossil records are scanty, Knaust (2010) reported some fossils and traces from Germany, dating back to the Middle Triassic Period. Fossils of the genus Amiskwia from the Burgess Shale in Canada were once thought to be ribbon worms. However, many paleontologists now believe they are not true ribbon worms because they lack key features such as a rhynchocoel and intestinal caeca.
Some other fossils of worm-like animals were reported by Knaust & Desrochers (2019), belonging to the Late Ordovician Period. These specimens had pointed or rounded anterior ends containing a rhynchocoel with a proboscis, characters that compelled these scientists to classify them as ribbon worms. However, these partly preserved fossils may also, with further research, turn out to be annelids or turbellarians (a group of flatworms).
Ribbon worms are found worldwide, with the majority of species found in the tropics and subtropics. Most of these worms live in marine environments ranging from intertidal to deep sea zones. They are usually found burrowed in sediments, between shells and stones, or attached to holdfasts of algae or some sedentary marine animal. However, around 12 species are known to live in freshwater habitats, and another 12 species can be found in terrestrial environments, such as cool, damp soil under decaying logs.
Argonemertes dendyi, a terrestrial ribbon worm, is native to Australia but is also found in Sao Miguel in the Azores, the British Isles, the Grand Canary Island, and the island of Hawaii. Similarly, members of the terrestrial genus Geonemertes are mostly found in Australasia; however, they are also spotted in Seychelles, Tristan da Cunha, Canary Islands, Madeira, and the Azores.
These worms are primarily carnivorous, feeding on small invertebrates like bivalves, polychaetes, and crustaceans. They also feed on various insects and myriapods (including their eggs) and even on living or dead fish. Some commensal worms, like those in the order Bdellonemertea, feed on phytoplankton captured from their hosts (usually clams and freshwater snails).
They occasionally scavenge on dead and decaying organic matter.
Just as the prey comes in range, members of the class Anopla evert their proboscis and coil the prey around it. They then spit out sticky toxins to immobilize the prey and then slurp it into their mouth.
In contrast, members of the class Enopla can gather prey only after coming in physical contact with it. They first employ their stylets and stab the prey repeatedly to inject toxins and digestive secretions. They then lap the prey using the proboscis and swallow it whole after partial digestion of its tissues.
Some ribbon worms, like Lineus longissimus, absorb organic matter through their skin, whereas others, like members of the family Malacobdellidae, are suspension feeders, consuming floating food particles.
They move slowly by using their external cilia and gliding on a trail of slime, often produced by the secretory glands on their heads for lubrication. Those larger in size move and burrow into sediments by undulations of their muscles (muscle waves). Aquatic ribbon worms, on the other hand, swim by undulating their muscles dorsoventrally.
Members of the suborder Monostilifera use their proboscis to anchor to a nearby object. By sticking it to the surface, they pull their bodies toward the object, allowing them to move from one place to another.
Although little is known about the longevity of ribbon worms, at least one species, Paranemertes peregrina, has been reported with a lifespan of around 18 months.
Most ribbon worms are gonochoric, with separate male and female sexes; however, all freshwater species are hermaphroditic (both male and female reproductive organs in a single individual). When these worms are ready to mate, their gonads enlarge and become hollow, forming a lumen between the gut and body wall. Specialized germinal cells in the testis and the ovary produce sperm and eggs, which eventually fill the lumen. The worms become highly active and cluster together, forming a mating mass, followed by spawning or release of eggs. Tactile and chemical cues trigger this entire sequence of events.
The eggs are usually released in the water or laid in burrows, tubes, cocoons, and gelatinous strings. Usually, the males fertilize these eggs externally; however, in some bathypelagic or deep sea species, fertilization occurs internally within the female’s body, followed by the birth of live young in some (viviparity).
The zygote undergoes determinate development (the fate of each cell is predictable in the process of division) and cleaves spirally. They either directly develop into juveniles or form a planuliform larva. These larvae are typically short-lived and lecithotrophic or yolky, sometimes feeding on plankton (planktotrophic) before developing into juveniles.
However, in some species, like those in the order Heteronemertea and the family Hubrechtiidae, the zygote develops into a pilidium larva, characterized by ear flaps pulled down with a mouth between the flaps and a gut that lies across the body. This larva feeds on unicellular algae and grows in size, bursting out of the larva body as juveniles.
Though these worms do not generally invest in parental care, ovoviviparous species protect till the birth of the young.
Some species, like Lineus sanguineus, undergo asexual reproduction through transverse fission. The fragments form mucous cysts, which regenerate into complete individuals.
Though soft-bodied, these worms surprisingly do not have many predators. They are preyed upon by seabirds, benthic fish, and a few invertebrates, like horseshoe crabs and other ribbon worms.
However, they act as predators of annelid worms, clams, crabs, and even small fish.
Their nerves contain hemoglobin and often appear pink in color. The presence of hemoglobin helps them fight anoxia (complete absence of oxygen) when burrowing in sediments lacking oxygen.
Like most invertebrates, these worms, too, play a crucial role in maintaining the food web, both as predators and prey. While most species are free-living, some are parasitic and influence the population size of their hosts. For example, members of the genus Carcinonemertes have led to about 55% egg mortality in the populations of Dungeness crabs (Cancer magister) in California, thereby causing a notable decline in their numbers.
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