Editing Echiura

{{Short description|Group of marine animals ("spoon worms")}}
{{good article}}
{{Automatic taxobox
|fossil_range={{fossil range|Darriwilian|Recent|ref=<ref>{{cite journal |last1=Botting |first1=Joseph |last2=Muir |first2=Lucy |title=Echiuran worms from the Middle Ordovician Castle Bank Biota of Wales, UK |journal=Acta Palaeontologica Polonica |date=2023 |volume=68 |doi=10.4202/app.01107.2023 |url=https://www.app.pan.pl/archive/published/app68/app011072023.pdf}}</ref>}}
|image=Urechiscaupo (cropped and mirrored).jpg
|image_caption=''[[Urechis caupo]]''
|taxon=Echiura
|authority=Newby, 1940<ref name=EncycloBrit/><ref name=WoRMS>{{cite WoRMS |author=Tanaka, Masaatsu |year=2017 |title=Echiura |id=1269 |access-date=17 February 2019}}</ref>
|subdivision_ranks=Subdivision
|subdivision=* Order [[Bonelliida]]
** Family [[Bonelliidae]]
** Family [[Ikedidae]]
* Order [[Echiurida]]
** Family [[Echiuridae]]
** Family [[Thalassematidae]]
** Family [[Urechidae]]
}}
[[File:Brockhaus-Efron Echiuridea 1.jpg|thumb|right|120px|''Echiurus'']]

The '''Echiura''', or '''spoon worms''', are a small group of [[ocean|marine]] [[animal]]s. Once treated as a separate [[phylum]], they are now considered to belong to [[Annelida]]. Annelids typically have their bodies divided into [[Segmentation (biology)|segments]], but echiurans have secondarily lost their segmentation. The majority of echiurans live in burrows in soft sediment in shallow water, but some live in rock crevices or under boulders, and there are also deep sea forms. More than 230 species have been described.<ref name=Zhang2011>{{cite journal|author=Zhang, Z.-Q.|title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness |journal=Zootaxa |volume=3148 |year=2011 |pages=7–12 |doi=10.11646/zootaxa.3148.1.3 |url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2022-10-09 |url-status=live}}</ref>
Spoon worms are cylindrical, soft-bodied animals usually possessing a non-retractable [[proboscis]] which can be rolled into a scoop-shape to feed. In some species the proboscis is ribbon-like, longer than the trunk and may have a forked tip. Spoon worms vary in size from less than a centimetre in length to more than a metre.

Most are deposit feeders, collecting [[detritus]] from the sea floor. [[Fossil]]s of these worms are seldom found and the earliest known fossil specimen is from the [[Darriwilian|Middle Ordovician]].

==Taxonomy and evolution==
The spoonworm ''[[Echiurus echiurus]]'' was first described by the Prussian naturalist [[Peter Simon Pallas]] in 1766; he placed it in the earth worm genus ''[[Lumbricus]]''.<ref>{{cite WoRMS |author=Tanaka, Masaatsu |year=2017 |title=''Echiurus echiurus'' (Pallas, 1766) |id=110377 |access-date=17 February 2019}}</ref>  In the mid-nineteenth century Echiura was placed, alongside [[Sipuncula]] and [[Priapulida]], in the now defunct class [[Gephyrea]] (meaning a "bridge") in Annelida, because it was believed that they provided a link between annelids and [[holothuria]]ns.<ref>{{cite journal | author1 = Banta, W.C. | author2 = Rice, M.E. | title = A restudy of the Middle Cambrian Burgess Shale fossil worm, ''Ottoia prolifica'' | journal = Proceedings of the International Symposium on the Biology of the Sipunculata and Echiura | volume = 11 | year = 1970 | url = http://si-pddr.si.edu/dspace/bitstream/10088/6842/1/sms_banta_1971.pdf | access-date = 2019-02-17 | archive-date = 2012-03-12 | archive-url = https://web.archive.org/web/20120312040447/http://si-pddr.si.edu/dspace/bitstream/10088/6842/1/sms_banta_1971.pdf | url-status = dead }}</ref> In 1898, Sedgwick raised the sipunculids and priapulids to phylum status but considered Echiuroids to be a class of the Annelida.<ref>{{cite web |url=http://onlinezoologists.com/cs/echiura |title=Phylum Echiura |author=Elsberry, Wesley R. |date=10 June 2006 |publisher=Online Zoologists |access-date=17 February 2019}}</ref> During the early 1900s, a biologist named Jon Stanton Whited devoted his working life to study the echiurans and classify many of its different species. In 1940, after the American marine biologist W. W. Newby had studied the embryology and development of ''[[Urechis caupo]]'', he raised the group to phylum status.<ref name=EncycloBrit>{{cite encyclopedia |url=https://www.britannica.com/animal/spoonworm |title=Spoon Worm |encyclopedia=Britannica.com |access-date=9 March 2019}}</ref>

They are now universally considered to represent derived [[annelid]] worms; as such, their ancestors were segmented worms but echiurans have secondarily lost their segmentation.<ref name="Dunn2008">{{Cite journal|last1=Dunn|first1=C. W.|last2=Hejnol|first2=A.|last3=Matus|first3=D. Q.|last4=Pang|first4=K.|last5=Browne|first5=W. E.|last6=Smith|first6=S. A.|last7=Seaver|first7=E.|last8=Rouse|first8=G. W.|last9=Obst|first9=M.|last10=Edgecombe|doi=10.1038/nature06614|first10=G. D.|last11=Sørensen|first11=M. V.|last12=Haddock|first12=S. H. D. |author12-link=Steven Haddock|last13=Schmidt-Rhaesa|first13=A.|last14=Okusu|first14=A.|last15=Kristensen|first15=R. M. B.|last16=Wheeler|first16=W. C.|last17=Martindale|first17=M. Q.|last18=Giribet|first18=G.|title=Broad phylogenomic sampling improves resolution of the animal tree of life |journal=Nature |volume=452|issue=7188|pages=745–749|year=2008 |pmid=18322464|bibcode=2008Natur.452..745D |s2cid=4397099}}</ref><ref name="Bourlat2008">{{Cite journal|last1=Bourlat|first1=S.|last2=Nielsen|first2=C.|last3=Economou|first3=A.|last4=Telford|first4=M.|title=Testing the new animal phylogeny: A phylum level molecular analysis of the animal kingdom|doi=10.1016/j.ympev.2008.07.008|journal=Molecular Phylogenetics and Evolution|volume=49|issue=1|pages=23–31|year=2008|pmid=18692145|bibcode=2008MolPE..49...23B }}</ref><ref name="Struck2011">{{Cite journal|last1=Struck|first1=T. H.|last2=Paul|first2=C.|last3=Hill|first3=N. |last4=Hartmann|first4=S.|last5=Hösel|first5=C.|last6=Kube|first6=M.|last7=Lieb|first7=B.|last8=Meyer|first8=A.|last9=Tiedemann|first9=R.|last10=Purschke|doi=10.1038/nature09864|first10=G. N.|last11=Bleidorn|first11=C.|title=Phylogenomic analyses unravel annelid evolution |journal=Nature |volume=471 |issue=7336|pages=95–98|year=2011|pmid=21368831|bibcode=2011Natur.471...95S |s2cid=4428998}}</ref><ref name="Struck">{{Cite journal|last1=Struck|first1=T. H.|last2=Schult |first2=N.|last3=Kusen|first3=T.|last4=Hickman|first4=E.|last5=Bleidorn|first5=C.|last6=McHugh|first6=D.|last7=Halanych|first7=K. M.|title=Annelid phylogeny and the status of Sipuncula and Echiura|doi=10.1186/1471-2148-7-57|journal=BMC Evolutionary Biology |volume=7 |page=57 |year=2007 |issue=1 |pmid=17411434|pmc=1855331 |doi-access=free |bibcode=2007BMCEE...7...57S }}</ref> Their presumed sister group is the [[Capitellidae]].<ref>{{cite journal |last1=Tilic |first1=Ekin |last2=Lehrke |first2=Janina |last3=Bartolomaeus |first3=Thomas |last4=Colgan |first4=Donald James |title=Homology and Evolution of the Chaetae in Echiura (Annelida) |journal=PLOS ONE |date=3 March 2015 |volume=10 |issue=3 |pages=e0120002 |doi=10.1371/journal.pone.0120002 |pmid=25734664 |pmc=4348511 |bibcode=2015PLoSO..1020002T |doi-access=free }}</ref>

Having no hard parts, these worms are seldom found as fossils. One of the oldest known unambiguous examples is ''[[Coprinoscolex ellogimus]]'' from the [[Mazon Creek fossil beds]] in Illinois, dating back to the [[Pennsylvanian (geology)#Subdivisions|Middle Pennsylvanian]] period. This exhibits a proboscis, cigar‐shaped body and convoluted gut, and shows that already at that time, echiurans were unsegmented and were essentially similar to modern forms.<ref name="Jones1977">{{Cite journal|last1=Jones|first1=D.|last2=Thompson|first2=I. D. A.|doi=10.1111/j.1502-3931.1977.tb00627.x|title=Echiura from the Pennsylvanian Essex Fauna of northern Illinois|journal=Lethaia|volume=10|issue=4|pages=317|year=1977|bibcode=1977Letha..10..317J }}</ref>  However, U-shaped [[burrow fossil]]s that could be Echiuran have been found dating back to the [[Cambrian]],<ref>{{cite web |url=https://ucmp.berkeley.edu/annelida/echiura.html |title=Introduction to the Echiura |publisher=UC Museum of Paleontology |access-date=7 March 2019}}</ref> and an Ordovician species of thalassematid named ''[[Llwygarua|Llwygarua suzannae]]'' was found in the [[Castle Bank]] lagerstätte.

==Anatomy==
Spoon worms vary in size from the giant ''[[Ikeda taenioides]]'', nearly {{convert|2|m|ft|0|abbr=on}} long with its proboscis extended, to the minute ''[[Lissomyema]]'', measuring just {{convert|1|cm|in|1|abbr=on}}.<ref name=Ruppert/> 
Their bodies are generally cylindrical with two wider regions separated by a narrower region. There is a large extendible, scoop-shaped [[proboscis]] in front of the mouth which gives the animals their common name. This proboscis resembles that of [[Sipuncula|peanut worms]] but it cannot be retracted into the body. It houses a brain and may be [[Homology (biology)|homologous]] to the [[prostomium]] of other annelids.<ref name=IZ>{{cite book |author=Barnes, Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |location=Philadelphia, PA|pages=870–873|isbn=0-03-056747-5}}</ref> The proboscis has rolled-in margins and a groove on the ventral surface. The distal end is sometimes forked. The proboscis can be very long; in the case of the Japanese species ''[[Ikeda taenioides]]'', the proboscis can be {{convert|150|cm}} long while the body is only {{convert|40|cm}}. Even smaller species like ''[[Bonellia (annelid)|Bonellia]]'' can have a proboscis a metre (yard) long. The proboscis is used primarily for feeding. Respiration takes place through the proboscis and the body wall, with some larger species also using [[cloaca]]l irrigation. In this process, water is pumped into and out of the rear end of the gut through the anus.<ref name=Ruppert/><ref>{{cite web |url=http://www.reefs.org/library/aquarium_net/0497/0497_4.html |title=Part 6: Phylum Sipuncula and Phylum Annelida |author=Toonen, Rob |year=2012 |work=Reefkeeper's Guide to Invertebrate Zoology |access-date=10 November 2012 |archive-url=https://web.archive.org/web/20120826014338/http://www.reefs.org/library/aquarium_net/0497/0497_4.html |archive-date=26 August 2012 |url-status=dead }}</ref>

Compared with other annelids, echiurans have relatively few [[seta]]e (bristles). In most species, there are just two, located on the underside of the body just behind the proboscis, and often hooked. In others, such as ''Echiurus'', there are also further setae near the posterior end of the animal. Unlike most annelids, adult echiurans have no trace of segmentation.<ref name=IZ/> Most echiurans are a dull grey or brown but a few species are more brightly coloured, such as the translucent green ''[[Listriolobus pelodes]]''.<ref name=Walls>{{cite book |title=Encyclopedia of Marine Invertebrates |last=Walls |first=Jerry G. |year=1982 |publisher=TFH Publications |isbn=0-86622-141-7 |pages=[https://archive.org/details/encyclopediaofma0000unse_f6h1/page/262 262&ndash;267] |url=https://archive.org/details/encyclopediaofma0000unse_f6h1/page/262 }}</ref>

The body wall is muscular. It surrounds a large [[coelom]] which leads to a long looped intestine with an anus at the rear tip of the body.<ref name=II>{{cite book |title=Intertidal Invertebrates of the Central California Coast |last=Felty Light |first=Sol |year=1954|page=108 |publisher=University of California Press |url=https://books.google.com/books?id=2Qnod-98-q8C&q=scutum+tergum&pg=PA129 |access-date=30 November 2011 |isbn=9780520007505 }}</ref> The intestine is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched [[diverticulum|diverticula]] are connected to the [[rectum]]. These are lined with numerous minute [[Cilium|ciliated]] funnels that open directly into the body cavity, and are presumed to be excretory organs.<ref name=IZ/> The proboscis has a small coelomic cavity separated from the main coelom by a [[septum]].<ref name=Ruppert/>

Echiurans do not have a distinct respiratory system, absorbing oxygen through the body wall of both the trunk and proboscis, and through the cloaca in ''Urechis''.<ref name=Ruppert/> Although some species lack a blood vascular system, where it is present, it resembles that of other annelids. The blood is essentially colourless, although some [[haemoglobin]]-containing cells are present in the [[Coelom#Coelomic fluid|coelomic fluid]] of the main body cavity. There can be anywhere from one to over a hundred [[metanephridium|metanephridia]] for excreting nitrogenous waste, which typically open near the anterior end of the animal.<ref name=IZ/> The nervous system consists of a brain near the base of the proboscis, and a ventral nerve cord running the length of the body. Aside from the absence of segmentation, this is a similar arrangement to that of other annelids. Echiurans do not have any eyes or other distinct sense organs,<ref name=IZ/> but the proboscis is presumed to have a tactile sensory function.<ref name=Walls/>

==Distribution and habitat==
[[Image:Ochetostoma erythrogrammon sujiyumusi.jpg|thumb|''[[Ochetostoma erythrogrammon]]'']]
Echiurans are exclusively marine and the majority of species live in the Atlantic Ocean. They are mostly [[infaunal]], occupying burrows in the seabed, either in the lower [[intertidal zone]] or the shallow [[Neritic zone|subtidal]] (e.g. the genera ''[[Echiurus]]'', ''[[Urechis]]'', and ''[[Ikeda (annelid)|Ikeda]]'').<ref name=Walls/> Others live in holes in coral heads, and in rock crevices. Some are found in deep waters including at [[Abyssal zone|abyssal]] depths; in fact more than half the 70 species in [[Bonelliidae]] live below {{convert|3000|m|ft|-3|abbr=on}}.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=490–495 }}</ref> They often congregate in sediments with high concentrations of organic matter. One species, ''[[Lissomyema mellita]]'', which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead [[sand dollar]]s. When the worm is very small, it enters the test and later becomes too large to leave.<ref>{{cite journal |author=Conn, H.W. |year=1886 |title=Life history of ''Thalassema mellita'' |journal=Stud. Biol. Lab. Johns Hopkins Univ.|pages=1884–1887 }}</ref>

In the 1970s, the spoon worm ''[[Listriolobus pelodes]]'' was found on the [[continental shelf]] off [[Los Angeles]] in numbers of up to 1,500 per square metre (11 square feet) near [[sewage]] outlets.<ref name=Stull/> The burrowing and feeding activities of these worms churned up and aerated the sediment and promoted a balanced [[ecosystem]] with a more diverse [[fauna]] than would otherwise have existed in this heavily [[Pollution|polluted]] area.<ref name=Stull>{{cite journal |author1=Stull, Janet K. |author2=Haydock, C.Irwin |author3=Montagne, David E. |year=1986 |title=Effects of ''Listriolobus pelodes'' (Echiura) on coastal shelf benthic communities and sediments modified by a major California wastewater discharge |journal=[[Estuarine, Coastal and Shelf Science]] |volume=22 |issue=1 |pages=1–17 |doi=10.1016/0272-7714(86)90020-X |bibcode=1986ECSS...22....1S }}</ref>

==Behaviour==
A spoon worm can move about on the surface by extending its proboscis and grasping some object before pulling its body forward. Some worms, such as ''[[Echiurus]]'', can leave the [[Substrate (biology)|substrate]] entirely, swimming by use of the proboscis and contractions of the body wall.<ref>{{Cite EB1911|wstitle= Echiuroidea |volume= 08 |last= Shipley |first= Arthur Everett |author-link= Arthur Everett Shipley | pages = 882&ndash;883 }}</ref>

Digging behaviour has been studied in ''[[Echiurus echiurus]]''. When burrowing, the proboscis is raised and folded backwards and plays no part in the digging process. The front of the trunk is shaped into a wedge and pushed forward, with the two anterior chaetae (hooked chitinous bristles) being driven into the sediment. Next the rear end of the trunk is drawn forward and the posterior chaetae anchor it in place. These manoeuvres are repeated and the worm slowly digs its way forwards and downwards. It takes about forty minutes for the worm to disappear from view. The burrow descends diagonally and then flattens out, and it may be a metre or so long before ascending vertically to the surface.<ref>{{cite web |url=https://inverts.wallawalla.edu/Echiura/Echiurus_echiurus.html |title=''Echiurus echiurus'' subspecies ''alaskanus'' Fisher, 1946 |author=Cowles, Dave |year=2005 |work=Invertebrates of the Salish Sea |access-date=18 February 2019}}</ref>

[[Image:Echiurien Thiladhoo.JPG|thumb|right|Characteristic forked proboscis of an echiurian worm in the Maldives]]
[[File:Bonélie (Bonellia viridis) PC301461.JPG|thumb|''[[Bonellia viridis]]'', female]]
Spoon worms are typically [[detritivore]]s, extending the flexible and mobile proboscis and gathering organic particles that are within reach. Some species can expand the proboscis by ten times its contracted length. The proboscis is moved by the action of cilia on the lower (ventral) surface "creeping" it forward. When food particles are encountered, the sides of the proboscis curl inward to form a ciliated channel.<ref name=Ruppert/>

A worm such as ''Echiurus'', living in the sediment, extends its proboscis from the rim of its burrow with the ventral side on the substrate. The surface of the proboscis is well equipped with [[mucus|mucous]] glands to which food particles adhere. The mucus is bundled into boluses by [[Cilium|cilia]] and these are passed along the feeding groove by cilia to the mouth. The proboscis is periodically withdrawn into the burrow and later extended in another direction.<ref name=Walls/>

''[[Urechidae]]'', another group of tube-dwellers, has become [[filter feeder]]s.<ref>[https://kmkjournals.com/upload/PDF/ArthropodaSelecta/27/27_4_319_324_Burukovsky_Marin_for_Inet.pdf The food composition of the symbiotic crab Pinnixa rathbunae Sakai, 1934 (Brachyura: Pinnotheridae) from burrows of the spoon worm Urechis unicinctus (von Drasche, 1881 (Echiurida: Urechidae) in Vostok Bay of the Sea of Japan]</ref> It has a short proboscis and a ring of mucous glands at the front of its body. It expands its muscular body wall to deposit a ring of mucus on the burrow wall then retreats backwards, exuding mucus as it goes and spinning a mucus net. It then draws water through the burrow by [[Peristalsis|peristaltic contractions]] and food particles stick to the net. When this is sufficiently clogged up, the spoon worm moves forward along its burrow devouring the net and the trapped particles. This process is then repeated and in a nutrient-rich area may take only a few minutes to complete. Large particles are squeezed out of the net and eaten by other [[invertebrate]]s living [[Commensalism|commensally]] in the burrow. These typically include a small [[crab]], a [[Polynoidae|scale worm]] and often a [[fish]] lurking just inside the back entrance.<ref name=Walls/>

''[[Ochetostoma erythrogrammon]]'' obtains its food by another method. it has two vertical burrows connected by a horizontal one. Stretching out its proboscis across the substrate it shovels material into its mouth before separating the edible particles. It can lengthen the proboscis dramatically while exploring new areas and periodically reverses its orientation in the burrow so as to use the back entrance to feed.<ref>{{cite journal |author=Chuang, S. H. |year=1962 |title=Feeding Mechanism of the Echiuroid, ''Ochetostoma erythrogrammon'' Leuckart & Rueppell, 1828 |journal=Biological Bulletin |volume=123 |issue=1 |pages=80&ndash;85 |doi=10.2307/1539504 |jstor=1539504 |url=https://www.biodiversitylibrary.org/part/37785 }}</ref> Other spoon worms live concealed in rock crevices, empty gastropod shells, sand dollar [[Test (biology)|tests]] and similar places, extending their proboscises into the open water to feed.<ref name=II/> Some are [[scavenger]]s or detritivores, while others are interface grazers and some are [[Filter feeder|suspension feeders]].<ref>{{cite WoRMS |author=van der Land, Jacob |year=2004 |title=Echiuroidea |id=110342 |access-date=30 November 2011}}</ref>

While the proboscis of a burrowing spoon worm is on the surface it is at risk of [[predation]] by bottom-feeding fish. In some species, the proboscis will [[Autotomy|autotomise]] (break off) if attacked and the worm will regenerate a proboscis over the course of a few weeks.<ref name=Walls/> In a study in California, one of the most commonly found dietary items of the [[leopard shark]] was found to be the tube-dwelling [[Urechis caupo|innkeeper worm]] (''Urechis caupo'') which it extracted from the sediment by suction.<ref>{{cite journal |author=Tallent, L.G. |year=1976 |title=Food habits of the leopard shark, Triakis semifasciata, in Elkhorn Slough, Monterey Bay, California |journal=California Fish and Game |volume=62 |issue=4 |pages=286–298 |url=https://www.cabdirect.org/cabdirect/abstract/19771453560 }}</ref>

==Reproduction==
Echiurans are [[dioecious]], with separate male and female individuals. The [[gonad]]s are associated with the [[peritoneum|peritoneal]] membrane lining the body cavity, into which they release the [[gamete]]s. The sperm and eggs complete their maturation in the body cavity, before being stored in genital sacs, which are specialised metanephridia. At spawning time, the genital sacs contract and the gametes are squeezed into the water column through pores on the worm's ventral surface. Fertilization is external.<ref name=IZ/>

Fertilization is internal in the sexual dimorphic order Bonelliida, which has dwarf males living inside the female. Members of the order Echiurida have external fertilization and are sexual monomorphic.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S1055790316000713 | doi=10.1016/j.ympev.2016.03.003 | title=A comprehensive molecular phylogeny of spoon worms (Echiura, Annelida): Implications for morphological evolution, the origin of dwarf males, and habitat shifts | journal=Molecular Phylogenetics and Evolution | date=June 2016 | volume=99 | pages=247–260 | last1=Goto | first1=Ryutaro | pmid=26975690 | bibcode=2016MolPE..99..247G }}</ref><ref>{{cite journal | pmc=3572977 | date=2013 | last1=Goto | first1=R. | last2=Okamoto | first2=T. | last3=Ishikawa | first3=H. | last4=Hamamura | first4=Y. | last5=Kato | first5=M. | title=Molecular Phylogeny of Echiuran Worms (Phylum: Annelida) Reveals Evolutionary Pattern of Feeding Mode and Sexual Dimorphism | journal=PLOS ONE | volume=8 | issue=2 | pages=e56809 | doi=10.1371/journal.pone.0056809 | doi-access=free | pmid=23457618 | bibcode=2013PLoSO...856809G }}</ref>

Fertilized eggs hatch into free-swimming [[trochophore]] larvae. In some species, the larva briefly develops a segmented body before transforming into the adult body plan, supporting the theory that echiurans evolved from segmented ancestors resembling more typical annelids.<ref name=IZ/>

The species ''[[Bonellia viridis]]'', also remarkable for the possible [[antibiotic]] properties of [[bonellin]], the green chemical in its skin, is unusual for its extreme [[sexual dimorphism]]. Females are typically {{convert|15|cm|in|0|abbr=on}} in body length, excluding the proboscis, but the males are only {{convert|1|to|3|mm|in|2|abbr=on}} long, and spend their adult lives within the uterus of the female.<ref name=IZ/>

==Culinary Use==
[[File:Echiura in Korea1.jpg|thumb|[[Urechis unicinctus|Spoon worms]] at a market in South Korea]]
Spoon worms are eaten in East and Southeast Asia. In South Korea [[Urechis unicinctus|fat innkeeper worms]] (''Urechis unicinctus'') are known as ''gaebul'' (개불). These worms are much prized and are often available at markets and stalls, chopped up and served raw in combination with raw [[sea cucumber]], [[sea squirt]] and [[sea urchin]], dressed with [[chili sauce]] and [[soy sauce]].<ref>{{cite book|author1=Brown, Nicholas |author2=Eddy, Steve|title=Echinoderm Aquaculture|url=https://books.google.com/books?id=pWHhCgAAQBAJ&pg=PA60 |year=2015 |publisher=Wiley |isbn=978-1-119-00585-8 |page=60}}</ref> They are also eaten as a fermented product known as ''gaebul-jeot''.<ref>{{cite book|author1=Kun-Young Park|author2=Dae Young Kwon|author3=Ki Won Lee|author4=Sunmin Park|title=Korean Functional Foods: Composition, Processing and Health Benefits|url=https://books.google.com/books?id=KU5WDwAAQBAJ&pg=PT232 |year=2018 |publisher=CRC Press|isbn=978-1-351-64369-6 |page=232}}</ref>

==List of families==
According to the [[World Register of Marine Species]]:<ref name=WoRMS/>
* suborder [[Bonelliida]]<ref>{{cite WoRMS |author=Tanaka, Masaatsu |year=2017 |title=Bonelliida |id=110344 |access-date=9 March 2019}}</ref>
** family [[Bonelliidae]] <small>Lacaze-Duthiers, 1858</small>
** family [[Ikedidae]] <small>Bock, 1942</small>
* suborder [[Echiurida]]<ref>{{cite WoRMS |author=Tanaka, Masaatsu |year=2017 |title=Echiurida |id=110345 |access-date=9 March 2019}}</ref>
** family [[Echiuridae]] <small>Quatrefages, 1847</small>
** family [[Thalassematidae]] <small>Forbes & Goodsir, 1841</small>
** family [[Urechidae]] <small>Fisher & Macginitie, 1928</small>

<gallery style="text-align:center;" mode="packed">
Image:Un ver de la famille des Bonelliidae.jpg|A worm of the family [[Bonelliidae]]
Image:Bonelliidae à identifier.jpg|''[[Ochetostoma erythrogrammon]]'', family [[Echiuridae]]
Image:Arhynchite hayaoi - ZooKeys-312-013-g001.jpeg|''[[Arhynchite hayaoi]]'', family [[Thalassematidae]]
Image:Urechiscaupo.jpg|''[[Urechis unicinctus]]'', family [[Urechidae]]
</gallery>

==References==
{{Reflist|2}}

==External links==
*{{Commons category-inline}}
*{{Wikispecies-inline}}

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[[Category:Echiurans| ]]
[[Category:Annelids]]
[[Category:Extant Pennsylvanian first appearances]]
[[Category:Protostome subclasses]]