Polychaete

The Polychaeta /ˌpɒlɪˈkiːtə/, also known as the bristle worms or polychaetes, are a paraphyletic[1] class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. As such, polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the lugworm (Arenicola marina) and the sandworm or clam worm Alitta.

Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extremely high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe Nereus at the bottom of the Challenger Deep, the deepest known spot in the Earth's oceans.[2] Only 168 species (less than 2% of all polychaetes) are known from fresh waters.[3]

Polychaetes
Temporal range: Cambrian (or earlier?) – present
"A variety of marine worms": plate from Das Meer by M.J. Schleiden (1804–1881).
"A variety of marine worms": plate from Das Meer by M.J. Schleiden (1804–1881).
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Polychaeta
Grube, 1850
Groups included
Cladistically included but traditionally excluded taxa

Chaetopteridae[1]

Description

Polychaetes are segmented worms, generally less than 10 cm (4 in) in length, although ranging at the extremes from 1 mm (0.04 in) to 3 m (10 ft), in Eunice aphroditois. They can sometimes be brightly coloured, and may be iridescent or even luminescent. Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called chaetae, project from the parapodia.[4]

However, polychaetes vary widely from this generalised pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches, including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism, and parasitism, requiring various modifications to their body structures.

The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision.[4]

The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.[4]

Internal anatomy and physiology

Polychaeta anatomy en
General anatomy of a polychaete

The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the body cavity. Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it's more continuous.

The mouth of polychaetes is located on the peristomium, the segment behind the prostomium, and varies in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis. The digestive tract is a simple tube, usually with a stomach part way along.

The smallest species, and those adapted to burrowing, lack gills, breathing only through their body surfaces. Most other species have external gills, often associated with the parapodia.

A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavities.[4]

The blood may be colourless, or have any of three different respiratory pigments. The most common of these is haemoglobin, but some groups have haemerythrin or the green-coloured chlorocruorin, instead.

The nervous system consists of a single or double ventral nerve cord running the length of the body, with ganglia and a series of small nerves in each segment. The brain is relatively large, compared with that of other annelids, and lies in the upper part of the head. An endocrine gland is attached to the ventral posterior surface of the brain, and appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots, statocysts, and numerous additional sensory nerve endings, most likely in involved with the sense of touch, also occur on the body.[4]

Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish "chloragogen" tissue, similar to that found in oligochaetes, which appears to function in metabolism, in a similar fashion to that of the vertebrate liver.[4]

The cuticle is constructed from cross-linked fibres of collagen and may be 200 nm to 13 mm thick. Their jaws are formed from sclerotised collagen, and their setae from sclerotised chitin.[5]

Ecology

Spirobrancheus giganteus
Christmas tree worms (Spirobranchus giganteus) from East Timor.

Polychaetes are extremely variable in both form and lifestyle, and include a few taxa that swim among the plankton or above the abyssal plain. Most burrow or build tubes in the sediment, and some live as commensals. A few are parasitic. The mobile forms (Errantia) tend to have well-developed sense organs and jaws, while the stationary forms (Sedentaria) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms.

Underwater polychaetes have eversible mouthparts used to capture prey.[6] A few groups have evolved to live in terrestrial environments, like Namanereidinae with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange.

Notable polychaetes
  • One notable polychaete, the Pompeii worm (Alvinella pompejana) is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are among the most heat-tolerant complex animals known.
  • A recently discovered genus, Osedax, includes a species nicknamed the "bone-eating snot flower".[7]
  • Another remarkable polychaete is Hesiocaeca methanicola, which lives on methane clathrate deposits.
  • Lamellibrachia luymesi is a cold seep tube worm that reaches lengths of over 3 m and may be the most long-lived animal, being over 250 years old.
  • A still unclassified multilegged predatory polychaete worm was identified only by observation from the underwater vehicle Nereus at the bottom of the Challenger Deep, the greatest depth in the oceans, near 10,902 m (35,768 ft) in depth. It was about an inch long visually, but the probe failed to capture it, so it could not be studied in detail.[8]
  • The Bobbit worm (Eunice aphroditois) is a predatory species that can achieve a length at 3 m (9.8 feet), with an average diameter of 25 mm (0.98 in).

Reproduction

Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of gonads in every segment, but most species exhibit some degree of specialisation. The gonads shed immature gametes directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species, or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species copulate, but most fertilize their eggs externally.

The fertilized eggs typically hatch into trochophore larvae, which float among the plankton, and eventually metamorphose into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg.[4]

Some polychaetes exhibit remarkable reproductive strategies. Some species in the genus Eunicie reproduce by epitoky. For much of the year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches, the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half, responsible for breeding, is known as the epitoke. Each of the epitoke segments is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed, and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water.[9]

A similar strategy is employed by the deep sea worm Syllis ramosa, which lives inside a sponge. The rear end of the worm develops into a "stolon" containing the eggs or sperm; this stolon then becomes detached from the parent worm and rises to the sea surface, where fertilisation takes place.[10]

Fossil record

Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte, a rich, sedimentary deposit in Greenland tentatively dated to the late Atdabanian (early Cambrian). The oldest found is Phragmochaeta canicularis.[11] Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid,[12] is now considered to represent a mollusc.[13][14] An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.[15][16]

Being soft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the mineralized tubes that some of them secrete.[17] Most important biomineralising polychaetes are serpulids, sabellids, and cirratulids. Polychaete cuticle does have some preservation potential; it tends to survive for at least 30 days after a polychaete's death.[5] Although biomineralisation is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the nonmineralised Burgess shale shows this need not always be the case.[5] Their preservation potential is similar to that of jellyfish.[5]

Taxonomy and systematics

Taxonomically, polychaetes are thought to be paraphyletic,[18] meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include the oligochaetes (earthworms and leeches), sipunculans, and echiurans. The Pogonophora and Vestimentifera were once considered separate phyla, but are now classified in the polychaete family Siboglinidae.

Much of the classification below matches Rouse & Fauchald, 1998, although that paper does not apply ranks above family.

Older classifications recognize many more (sub)orders than the layout presented here. As comparatively few polychaete taxa have been subject to cladistic analysis, some groups which are usually considered invalid today may eventually be reinstated.

These divisions were shown to be mostly paraphyletic in recent years.

See also

References

Bibliography

  • Campbell, Reece, and Mitchell. Biology. 1999.
  • Rouse, Greg W.; Fauchald, Kristian (1998). "Recent views on the status, delineation, and classification of the Annelida" (PDF). American Zoologist. 38 (6): 953–964. doi:10.1093/icb/38.6.953.

Notes

  1. ^ a b Struck, T. H.; Paul, C.; Hill, N.; Hartmann, S.; Hösel, C.; Kube, M.; Lieb, B.; Meyer, A.; Tiedemann, R.; Purschke, G. N.; Bleidorn, C. (2011). "Phylogenomic analyses unravel annelid evolution". Nature. 471 (7336): 95–98. Bibcode:2011Natur.471...95S. doi:10.1038/nature09864. PMID 21368831.
  2. ^ Geography of Guam ns.gov.gu Accessed Oct. 8, 2009
  3. ^ Glasby, Cristopher; Timm, Tarmo (2008). E. V. Balian; C. Lévêque; H. Segers; K. Martens (eds.). "Global diversity of polychaetes (Polychaeta: Annelida) in freshwater". Hydrobiologia. 595 (1: Freshwater Animal Diversity Assessment): 107–115. CiteSeerX 10.1.1.655.4467. doi:10.1007/s10750-007-9008-2.
  4. ^ a b c d e f g Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 469–525. ISBN 978-0-03-056747-6.
  5. ^ a b c d Briggs, Derek E. G.; Kear, Amanda J. (8 February 2016). "Decay and preservation of polychaetes: taphonomic thresholds in soft-bodied organisms". Paleobiology. 19 (1): 107–135. doi:10.1017/S0094837300012343. JSTOR 2400774.
  6. ^ "Bristleworm". MESA.
  7. ^ "'Zombie worms' found off Sweden". BBC News. 18 October 2005. Retrieved 12 February 2010.
  8. ^ Accessed Oct. 8, 2009 Geography of the ocean floor near Guam with some notes on exploration of the Challenger Deep.
  9. ^ Piper, Ross (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press.
  10. ^ Frost, Emily; Waters, Hannah (1 July 2015). "Some polychaetes have sex lives out of a science fiction movie". 14 fun facts about marine bristle worms. Smithsonian.com. Retrieved 9 August 2017.
  11. ^ Morris, S. C.; Peel, J. S. (2008). "The Earliest Annelids: Lower Cambrian Polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland". Acta Palaeontologica Polonica. 53: 137–148. doi:10.4202/app.2008.0110.
  12. ^ Butterfield, N. J. (1990). "A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa Walcott". Paleobiology. 16 (3): 287–303. doi:10.1017/S0094837300010009. JSTOR 2400789.
  13. ^ Smith, M. R. (2012). "Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: Implications for the ancestral molluscan radula". Proceedings of the Royal Society B. 279 (1745): 4287–4295. doi:10.1098/rspb.2012.1577. PMC 3441091. PMID 22915671.
  14. ^ Smith, M. R. (2014). "Ontogeny, morphology and taxonomy of the soft-bodied Cambrian 'mollusc' Wiwaxia". Palaeontology. 57 (1): 215–229. doi:10.1111/pala.12063.
  15. ^ Miller, A. J. (2004). A revised morphology of Cloudina with ecological and phylogenetic implications. CiteSeerX 10.1.1.526.5035.
  16. ^ Vinn, Olev; Zatoń, Michał (March 2012). "Inconsistencies in proposed annelid affinities of early biomineralized organism Cloudina (Ediacaran): structural and ontogenetic evidences". Carnets de géologie (Notebooks on geology) (Lettres). doi:10.4267/2042/46095.
  17. ^ Vinn, O; Mutvei, H (2009). "Calcareous tubeworms of the Phanerozoic". Estonian Journal of Earth Sciences. 58 (4): 286. doi:10.3176/earth.2009.4.07.
  18. ^ Westheide, W. (1997). "The direction of evolution within the Polychaeta". Journal of Natural History. 31 (1): 1–15. doi:10.1080/00222939700770011.

External links

Alvinella pompejana

Alvinella pompejana, the Pompeii worm, is a species of deep-sea polychaete worm (commonly referred to as "bristle worms"). It is an extremophile found only at hydrothermal vents in the Pacific Ocean, discovered in the early 1980s off the Galápagos Islands by French marine biologists.

Biomineralising polychaete

Biomineralising polychaetes are polychaetes that biomineralize.

The most important biomineralizing polychaetes are serpulids, sabellids and cirratulids. They secrete tubes of calcium carbonate. Serpulids have most advanced biomineralization system among the annelids. Serpulids possess very diverse tube ultrastructures. Serpulid tubes are composed of aragonite, calcite or mixture of both polymorphs. In addition to the tubes, some serpulid species secrete calcareous opercula. Some sabellids and cirratulids can secrete aragonitic tubes. Sabellid and cirratulid tubes have a spherulitic prismatic ultrastructure. There are thin organic sheets in serpulid tube mineral structures. These sheets have evolved as an adaptation to strengthen the mechanical properties of the tubes.

Burgessochaeta

Burgessochaeta is an extinct genus of polychaete annelids from the Middle Cambrian. Its fossils have been found in the Burgess Shale in British Columbia, Canada. 189 specimens of Burgessochaeta are known from the Greater Phyllopod bed, where they comprise 0.36% of the community. Specimens have also been found at Marble Canyon. The genus was described by Conway Morris (1979) and re-examined by Eibye-Jacobsen (2004).

Canadia (annelid)

Canadia (meaning of Canada or after Canada) is a genus of extinct annelid worm present in Burgess Shale type Konservat-Lagerstätte. It is found in strata dating back to the Delamaran stage of the Middle Cambrian around 505 million years ago, during the time of the Cambrian explosion. It was about 3 centimeters (1.2 inches) in length. Charles Doolittle Walcott named Canadia in 1911 after Canada, the country from which its remains have been found. 28 specimens of Canadia (annelid) are known from the Greater Phyllopod bed, where they comprise 0.05% of the community.

Canalipalpata

Canalipalpata, also known as bristle-footed annelids or fan-head worms, is an infraclass of polychaete worms, with 31 families in it including the Sabellida (tubeworms, fanworms, and feather duster worms) and the Alvinellidae, a family of deep-sea worms associated with hydrothermal vents.

The Canalipalpata have no teeth or jaws. Most are filter feeders. They have grooved palpi which are covered in cilia. These cilia are used to transport food particles to the mouth. However, the cilia and grooves have been lost in the Siboglinidae family.

Chaetopterus

Chaetopterus or the parchment worm or parchment tube worm is a genus of marine polychaete worm that lives in a tube it constructs in sediments or attaches to a rocky or coral reef substrate. The common name arises from the parchment-like appearance of the tubes that house these worms. Parchment tube worms are filter feeders and spend their adult lives in their tubes, unless the tube is damaged or destroyed. They are planktonic in their juvenile forms, as is typical for polychaete annelids. Species include the recently discovered deep water Chaetopterus pugaporcinus and the well-studied Chaetopterus variopedatus.

Errantia

Errantia, occasionally Aciculata, is a subclass of polychaete worms. These worms are found worldwide in marine environments and brackish water.

Glycera (annelid)

The genus Glycera is a group of polychaetes (bristle worms) commonly known as bloodworms. They are typically found on the bottom of shallow marine waters, and some species (e.g. common bloodworms) can grow up to 35 centimetres (14 in) in length.

Life That Glows

Life That Glows is a 2016 British nature documentary programme made for BBC Television, first shown in the UK on BBC Two on 9 May 2016. The programme is presented and narrated by Sir David Attenborough.

Life That Glows films the biology and ecology of bioluminescent organisms, that is, capable of creating light. The programme features fireflies, who use light as a means of sexual attraction, luminous fungi, luminous marine bacteria responsible for the Milky seas effect, the flashlight fish, the aposematism of the Sierra luminous millipede, earthworms, the bioluminescent tides created by blooms of dinoflagellates in Tasmania, as well as dolphins swimming in the bloom in the Sea of Cortez, the defensive flashes of brittle stars and ostracods, sexual attraction in ostracods, prey attraction by luminous click beetles in Cerrado,Brazil and the Arachnocampa gnats in New Zealand.

The programme then introduces many luminous deep sea animals, including the vampire squid, the polychaete worm Tomopteris that generates yellow light, the jellyfish Atolla, the comb jelly Beroe, the viper fish, pyrosomes, a dragonfish, and the polychaete worm Flota. Then, the programme discusses specialised adaptations in the eyes of particular animals to see bioluminescence, such as the barreleye fish and the cock-eyed squid. Lastly, they feature the mass spawning event of the firefly squid in Japan.

Nereididae

Nereididae (formerly spelled Nereidae) are a family of polychaete worms. It contains about 500 – mostly marine – species grouped into 42 genera. They may be commonly called ragworms or clam worms.

Orseis

For the polychaete worm genus, see Orseis (polychaete).In Greek mythology, Orseïs (; Ancient Greek: Ὀρσηΐς, derived from ὄρσω - orsô, "to rouse, stir, awaken, excite or arise") was the water-nymph (Naiad) of a spring in Thessalia, Greece, and the mythical ancestor of the Greeks. In some accounts, she was described as a mountain nymph (oread).

Palpata

Palpata is a subclass of polychaete worm. Members of this subclass are mostly deposit feeders on marine detritus or filter feeders. Palpata has become superfluous with the elevation of Canalipalpata to subclass.

Paralvinella sulfincola

Paralvinella sulfincola, also known as the sulfide worm, is a species of polychaete worm of the Alvinellidae family that thrives on undersea hot-water vents. It dwells within tubes in waters surrounding hydrothermal vents, in close proximity to super-heated fluids reaching over 300 °C. The upper thermal limit for this polychaete is unknown; however, it is unlikely they can survive in constant temperatures over 50 °C. It may tentatively be named a metazoan extremophile or, more specifically, a thermophile.

Their unique abilities to withstand high temperatures close to hydrothermal fluids enables them to prey upon sulphur-oxidising bacterial mats which grow close to the metal rich vent plume.

Parapodium

The term parapodium (Gr. para, beyond or beside + podia, feet) refers to two different organs. In annelids, parapodia are paired, un-jointed lateral outgrowths that bear the chaetae. In several groups of sea snails and sea slugs, 'parapodium' refers to lateral fleshy protrusions.

Phyllodocida

Phyllodocida is an order of polychaete worms in the subclass Aciculata. These worms are mostly marine though some are found in brackish water. Most are active benthic creatures, moving over the surface or burrowing in sediments, or living in cracks and crevices in bedrock. A few construct tubes in which they live and some are pelagic, swimming through the water column. There are estimated to be about 3,500 species in the order.

Pyramidelloidea

Pyramidelloidea is a superfamily of mostly very small sea snails, marine gastropod mollusks and micromollusks within the clade Panpulmonata.This is a voluminous taxon: above the species level close to 400 named taxa are referred to this gastropod superfamily.

Pyramidelloidea has both fossil and recent members. They live as ectoparasites on bivalve molluscs and polychaete worms, and have a sharp, piercing stylet instead of a radula.

Spirorbis

Spirorbis is a genus of very small (2–5 mm (0.079–0.197 in)) polychaete worms, usually with a white coiled shell. Members of the genus live in the lower littoral and sublittoral zones of rocky shores. Spirorbis worms usually live attached to seaweeds, but some species live directly on rocks, shells or other hard substrates. Spirorbis was once thought to have a fossil record extending back into the Early Paleozoic, but now all pre-Cretaceous spirorbins are known to be microconchids. The earliest members of genus appeared in the Miocene, but Oligocene finds may

also be possible. The genus contains the following species:

Spirorbis borealis Daudin, 1800

Spirorbis corallinae De Silva and Knight-jones, 1962

Spirorbis cuneatus Gee, 1964

Spirorbis granulatus

Spirorbis incongruus

Spirorbis inornatus L'hardy and Quievreux, 1962

Spirorbis knightjonesi Desilva, 1965

Spirorbis lineatus

Spirorbis marioni (Caullery and Mesnil, 1897)

Spirorbis medius

Spirorbis moerchi

Spirorbis nakamurai

Spirorbis quadrangularis

Spirorbis quasimilitaris Bailey, 1970

Spirorbis rupestris Gee and Knight-jones, 1962

Spirorbis semidentatus

Spirorbis similis

Spirorbis spirorbis (Linnaeus, 1758)

Spirorbis steueri Sterzinger, 1909

Spirorbis tridentata Levinsen, 1883

Spirorbis variabilis

Spirorbis violaceus

Syllidae

Syllidae is a family of small to medium-sized polychaete worms. Syllids are distinguished from other polychaetes by the presence of a muscular region of the anterior digestive tract known as the proventricle.Syllid worms range in size from 2–3 mm (0.08–0.12 in) to 14 centimetres (5.5 in). Most syllids are benthic organisms that transition to a pelagic epitoke for reproduction. They are found in all regions of the ocean, from the intertidal zone to the deep sea, and are especially abundant in shallow water.They are found in a range of habitats, moving actively on rock and sandy substrates, hiding in crevices and among seaweeds, and climbing on sponges, corals, hydrozoans, seagrasses and mangroves. They are generalist feeders.One species in the family, Syllis ramosa, was the first polychaete discovered to have a branching body plan.

Worm

Worms are many different distantly related animals that typically have a long cylindrical tube-like body and no limbs. Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms), 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi, and 58 metres (190 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land, but instead, live in marine or freshwater environments, or underground by burrowing.

In biology, "worm" refers to an obsolete taxon, vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word wyrm. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids (earthworms and marine polychaete or bristle worms), nematodes (roundworms), platyhelminthes (flatworms), marine nemertean worms ("bootlace worms"), marine Chaetognatha (arrow worms), priapulid worms, and insect larvae such as grubs and maggots.

Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms) which reside in the intestines of their host. When an animal or human is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Lungworm is also a common parasitic worm found in various animal species such as fish and cats.

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