Editing Sipuncula

{{Short description|Class of unsegmented annelids}}
{{good article}}
{{Use dmy dates|date=April 2017}}
{{Automatic taxobox
| image = Thysanocardia nigra.jpg
| image_caption = ''[[Thysanocardia nigra]]''
| fossil_range = {{geological range|Cambrian|Recent}}
| taxon = Sipuncula
| authority = [[Constantine Samuel Rafinesque|Rafinesque]], 1814
| subdivision_ranks = [[Class (biology)|Subclasses]], [[Order (biology)|orders]] and [[Family (biology)|families]]
| subdivision = *Subclass [[Phascolosomatidea]]
**Order Aspidosiphonida
***Family [[Aspidosiphonidae]]
**Order Phascolosomatida
***Family [[Phascolosomatidae]]
*Subclass [[Sipunculidea]]
**Order [[Golfingiida]]
***Family [[Golfingiidae]]
***Family [[Phascolionidae]]
***Family [[Sipunculidae]]
***Family [[Themistidae]]
}}

The '''Sipuncula''' or '''Sipunculida''' (common names '''sipunculid worms''' or '''peanut worms''') is a class containing about 162 [[species]] of [[segmentation (biology)|unsegmented]] [[sea|marine]] [[Annelid|annelid worms]]. Sipuncula was once considered a [[phylum]], but was demoted to a class of Annelida, based on recent molecular work.<ref name="Shen2009"/>

Sipunculans vary in size but most species are under {{convert|10|cm|in|0|abbr=on}} in length. The body is divided into an unsegmented, bulbous trunk and a narrower, [[Anatomical terms of location#Anterior and posterior|anterior]] section, called the "introvert", which can be retracted into the trunk. The mouth is at the tip of the introvert and is surrounded in most groups by a ring of short tentacles. With no hard parts, the body is flexible and mobile. Although found in a range of habitats throughout the world's oceans, the majority of species live in shallow water habitats, burrowing under the surface of sandy and muddy substrates. Others live under stones, in rock crevices or in other concealed locations.

Most sipunculans are deposit feeders, extending the introvert to gather food particles and draw them into the mouth, and retracting the introvert when feeding conditions are unsuitable or danger threatens. With a few exceptions, reproduction is sexual and involves a [[plankton]]ic larval stage. Sipunculid worms are used as food in some countries in south-east Asia.

{{anchor|Name|Etymology}}
==Taxonomy==
{{See also|List of bilaterial animal orders}}
{{lang|la|Sipuncula}} is a [[feminine (grammar)|feminine]] variant of the now-obsolete [[genus name]] {{lang|la|[[Sipunculus]]}}, itself a variant of the [[Latin language|Latin]] {{lang|la|{{linktext|siphunculus}}}} ("little tube"),<ref>{{cite web |url=https://en.oxforddictionaries.com/definition/sipuncula |title=Sipuncula |work=Oxford Living Dictionaries |publisher=Oxford University Press |access-date=10 February 2019 |archive-url=https://web.archive.org/web/20190212011749/https://en.oxforddictionaries.com/definition/sipuncula |archive-date=12 February 2019 |url-status=dead }}</ref> a [[Diminutive (linguistics)|diminutive]] of {{lang|la|sipho}} from [[Ancient Greek language|Greek]] {{lang|grc|{{linktext|σίφων}}}} (''síphōn'', "[[tube (fluid conveyance)|tube]], [[pipe (fluid conveyance)|pipe]]").

The Swedish naturalist [[Carl Linnaeus]] first described the worm {{lang|la|[[Sipunculus nudus]]}} in his {{lang|la|Systema Naturae}} in 1767.<ref>{{cite WoRMS |author=Saiz-Salinas, José |year=2009 |title=''Sipunculus (Sipunculus) nudus'' Linnaeus, 1766 |id=136084 |access-date=27 February 2019}}</ref> In 1814, the French zoologist [[Constantine Samuel Rafinesque]] used the word "Sipuncula" to describe the family (now [[Sipunculidae]]),<ref name="Rafinesque">{{cite book|author=Rafinesque, Constantine Samuel |title=Précis des découvertes et travaux somiologiques de m.r C. S. Rafinesque-Schmaltz entre 1800 et 1814 ou Choix raisonné de ses principales découvertes en zoologie et en botanique|url=https://archive.org/details/prcisdesdcouvert00rafi |year=1814 |publisher=Royale typographie militaire, aux dépens de l'auteur|page=[https://archive.org/details/prcisdesdcouvert00rafi/page/32 32]}}</ref> and in time, the term came to be used for the whole class.<ref>{{cite WoRMS  |year=2018 |title=Sipuncula |id=1268 |access-date=27 February 2019}}</ref> This is a relatively understudied group, and it is estimated there may be around 162 species worldwide.<ref name=Pancucci>{{cite web |url=https://okeanos-dspace.hcmr.gr/bitstream/handle/123456789/114/Monograph_02_The_Phylum_Sipuncula.pdf?sequence=1 |title=The phylum Sipuncula in the Mediterranean Sea |author1=Pancucci-Papadopoulou, M.A. |author2=Murina, G.V.V. |author3=Zenetos, A. |year=2014 |publisher=National Centre for Marine Research |access-date=10 February 2019 |archive-url=https://web.archive.org/web/20190212130746/https://okeanos-dspace.hcmr.gr/bitstream/handle/123456789/114/Monograph_02_The_Phylum_Sipuncula.pdf?sequence=1 |archive-date=12 February 2019 |url-status=dead }}</ref>

The [[phylogenetics|phylogenetic]] placement of this group in the past has proved troublesome. Originally classified as [[annelid]]s, despite the complete lack of segmentation, [[bristle]]s and other annelid characters, the phylum Sipuncula was later allied with the [[Mollusca]], mostly on the basis of [[morphogenesis|development]]al and [[larva]]l characters. These phyla have been included in a larger group, the [[Lophotrochozoa]], that also includes the [[annelid]]s, the [[nemertea|ribbon worms]] and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.<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 | date = 3 March 2011 | pmid =  21368831| bibcode = 2011Natur.471...95S | s2cid = 4428998 }}</ref><ref name="Hausdorf2007">{{cite journal | last1 = Hausdorf | first1 = B. | display-authors = etal   | year = 2007 | title = Spiralian Phylogenomics Supports the Resurrection of Bryozoa Comprising Ectoprocta and Entoprocta | journal = Molecular Biology and Evolution | volume = 24 | issue = 12| pages = 2723–2729 | doi=10.1093/molbev/msm214 | pmid=17921486| doi-access = free }}</ref> Subsequent analysis of the [[mitochondrion]]'s DNA has confirmed their close relationship to the Annelida (including [[echiura]]ns and [[Siboglinidae|pogonophorans]]).<ref name="Shen2009">{{Cite journal | doi = 10.1186/1471-2164-10-136| title = A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta| journal = BMC Genomics| volume = 10| pages = 136| year = 2009| last1 = Shen | first1 = X. | last2 = Ma | first2 = X. | last3 = Ren | first3 = J. | last4 = Zhao | first4 = F. | pmid=19327168 | pmc=2667193| doi-access = free}}</ref> It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.<ref>{{cite journal |first1=Andreas |last1=Wanninger |first2=Alen |last2=Kristof |first3=Nora |last3=Brinkmann |title=Sipunculans and segmentation |journal=Communicative and Integrative Biology |date=Jan–Feb 2009 |volume=2 |issue=1 |pages=56–59 |pmc=2649304 |pmid=19513266 |doi=10.4161/cib.2.1.7505}}</ref>

==Anatomy==
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 | footer            = A sipunculan with introvert everted (left) and retracted
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Sipunculans are worms ranging from {{convert|2|to|720|mm|in|1|abbr=on}} in length, with most species being under {{convert|10|cm|in|0|abbr=on}}. The sipunculan body is divided into an unsegmented, bulbous trunk and a narrower, anterior section, called the "introvert". Sipunculans have a body wall somewhat similar to that of most other [[annelid]]s (though unsegmented) in that it consists of an [[epidermis]] without [[Cilium|cilia]] overlain by a [[cuticle]], an outer layer of circular and an inner layer of longitudinal [[muscle|musculature]]. The body wall surrounds the [[body cavity|coelom]] (body cavity) that is filled with fluid on which the body wall musculature acts as a [[hydrostatic skeleton]] to extend or contract the animal. When threatened, Sipunculid worms can contract their body into a shape resembling a [[peanut]] [[seed|kernel]]—a practice that has given rise to the name "peanut worm". The introvert is pulled inside the trunk by two pairs of retractor muscles that extend as narrow ribbons from the trunk wall to attachment points in the introvert. It can be protruded from the trunk by contracting the muscles of the trunk wall, thus forcing the fluid in the body cavity forwards.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 863–870|isbn= 978-0-03-056747-6}}</ref> The introvert can vary in size from half the length of the trunk to several times its length, but whatever their comparative sizes, it is fully retractable.<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=495–501 }}</ref>

The mouth is located at the anterior end of the animal; in the subclass [[Sipunculidea]], the mouth is surrounded by a mass of 18 to 24 [[Cilium|ciliated]] [[tentacle]]s, while in the subclass [[Phascolosomatidea]], the tentacles are arranged in an arc above the mouth, surrounding the [[nuchal organ]], also located at the tip of the introvert. The tentacles each have a deep groove along which food is moved to the mouth by cilia.<ref name=Ruppert/> They are used to gather organic detritus from the water or substrate, and probably also function as [[gill]]s. In the family [[Themistidae]] the tentacles form an elaborate crown-like structure, the members of this group being specialized [[filter feeder]]s, unlike the other groups of sipunculans which are [[Detritivore|deposit feeders]].<ref name=Cutler>{{cite book|author=Cutler, Edward Bayler |title=The Sipuncula: Their Systematics, Biology, and Evolution|url=https://books.google.com/books?id=CIuxgRI-28kC&pg=PA5 |year=1994 |publisher=Cornell University Press|isbn=978-0-8014-2843-2 |page=5}}</ref> The tentacles are hollow and are extended via hydrostatic pressure in a similar manner as the introvert, but have a different mechanism from that of the rest of the introvert, being connected, via a system of ducts, to one or two contractile sacs next to the [[oesophagus]].<ref name=IZ/> Hooks are often present near the mouth on the introvert. These are proteinaceous, non-[[chitin]]ous specializations of the [[Epidermis (zoology)|epidermis]], either arranged in rings or scattered.<ref name=Ruppert/> They may be involved in scraping algae off rock, or alternatively provide anchorage.<ref name=Study/>

Three genera (''Aspidosiphon'', ''Lithacrosiphon'' and ''Cloeosiphon'') in the [[Aspidosiphonidae]] family possess epidermal structures, known as anal and caudal shields. These are patches of thickened, hard plates, and are used for boring into rock; the anal shield is near the anteriorly-located anus on the trunk, just below the introvert of the animal, while the caudal shield is at the posterior of the body.<ref name=Study>{{cite book|author=Australian Biological Resources Study|title=Polychaetes & Allies: The Southern Synthesis|url=https://books.google.com/books?id=Rin4l7QZ1YEC&pg=PA378 |year=2000 |publisher=Csiro Publishing |isbn=978-0-643-06571-0 |page=378}}</ref> In ''Aspidosiphon'' and ''Lithacrosiphon'' the anal shield is restricted to the dorsal side, causing the introvert to emerge at an angle, whereas it surrounds the anterior trunk in ''Cloeosiphon'' with the introvert emerging from its center. In ''Aspidosiphon'' the shield is a hardened, horny structure; in ''Lithacrosiphon'' it is a calcareous cone; in ''Cloeosiphon'' it is composed of separate plates. When the introvert is retracted in these animals, the anal shield blocks the entrance to its burrow. At the [[Anatomical terms of location#Anterior and posterior|posterior]] end of the trunk, a hardened caudal shield is sometimes present in ''Aspidosiphon'';<ref name=Schulze2006>{{Cite journal|author= Schulze, A. |title=Reconstructing the phylogeny of the Sipuncula |date=March 2005 |journal=Hydrobiologia |volume=535/536 |pages=277–296|doi= 10.1007/s10750-004-4404-3|display-authors= 1|last2= Cutler|first2= Edward B.|last3= Giribet|first3= Gonzalo|issue=1 |bibcode=2005HyBio.535..277S |s2cid=20948259 }}</ref> this may help with anchoring the animal in its burrow or may be used in the boring process.<ref name=Ruppert/>

===Digestive system===
The [[digestive tract]] of sipunculans starts with the esophagus, located between the introvert retractor muscles. In the trunk the intestine runs posteriorly, forms a loop and turns anteriorly again. The downward and upward sections of the gut are coiled around each other, forming a [[double helix]]. At the termination of the gut coil, the [[rectum]] emerges and ends in the [[anus]], located in the anterior third of the trunk. Digestion is [[Extracellular digestion|extra-cellular]], taking place in the lumen of the intestine. A rectal caecum, present in most species, is a blind ending sac at the transition between intestine and rectum with unknown function. The anus is often not visible when the introvert is retracted into the trunk.<ref name=IZ/>

===Circulation===
Sipunculans do not have a vascular [[blood]] system. Fluid transport and gas exchange are instead accomplished by the [[coelom]], which contains the respiratory pigment [[haemerythrin]], and the separate tentacular system, the two being separated by an elaborate [[septum]]. The [[Coelom#Coelomic fluid|coelomic fluid]] contains five types of coelomic cells: [[Blood cell|haemocytes]], [[granulocyte]]s, large multinuclear cells, ciliated urn-shaped cells and immature cells. The ciliated urn cells may also be attached to the [[peritoneum]] and assist in waste filtering from the coelomic fluid. Nitrogenous waste is excreted through a pair of [[nephridium|metanephridia]] opening close to the anus, except in ''Phascolion'' and ''Onchnesoma'', which have only a single nephridium.<ref name=IZ/> A ciliated funnel, or nephrostome, opens into the coelomic cavity at the anterior end, close to the nephridiopore. The metanephridia have an osmoregulation function  but it is unclear whether the mechanism is via filtration or secretion. They also serve as gamete storage and maintenance organs.<ref name=Ruppert/>

The tentacular coelom connects the tentacles at the tip of the introvert to a ring canal at their base, from which a contractile vessel runs along beside the esophagus and ends blindly posteriorly. Some evidence points towards the involvement of these structures in ultrafiltration.<ref name=Pilger>{{Cite journal|author1=Pilger, J.F. |author2=Rice, M.E. |title=Ultrastructural evidence for the contractile vessel of sipunculans as a possible ultrafiltration site |year=1987 |journal=American Zoology |volume=27 |pages=810a}}</ref> In crevice-dwelling sipunculans, respiration is mainly through the tentacular system, with oxygen diffusing into the trunk coelom from the tentacular coelom. However, in other species the skin is thin and respiration is mainly through the cuticle of the trunk, where oxygen uptake is assisted by the presence of dermal coelomic canals just beneath the [[epidermis]].<ref name=Ruppert/><ref name=ABRS>{{cite book|author=Australian Biological Resources Study|title=Polychaetes & Allies: The Southern Synthesis|url=https://books.google.com/books?id=Rin4l7QZ1YEC&pg=PA383 |year=2000 |publisher=Csiro Publishing |isbn=978-0-643-06571-0 |page=383}}</ref>

===Nervous system===
[[File:Esquema do Sistema Nervoso de Sipuncula.png|thumb|Nervous system of Sipuncula]]
The nervous system consists of  dorsal cerebral ganglion, brain above the oesophagus and a nerve ring around the oesophagus, which links the brain with the single ventral nerve cord that runs the length of the body. Lateral nerves lead off this to innervate the muscles of the body wall.<ref name=Ruppert/>

In some species, there are simple light-sensitive [[ocelli]] associated with the brain. Two organs, likely functioning as a unit for [[Chemoreceptor|chemoreception]] are located near its anterior margin; the non-ciliated cerebral organ, which possesses [[Bipolar neuron|bipolar sensory cells]], and the nuchal organ, located posterior to the brain.<ref name=IZ/> Similar light-sensing tubes have been reported in the [[Fauveliopsidae|fauveliopsid]] annelids.<ref>{{cite journal|year=2011|author= Purschke, Günter|title=Sipunculid-like ocellar tubes in a polychaete, ''Fauveliopsis cf. adriatica'' (Annelida, Fauveliopsidae): implications for eye evolution| journal=Invertebrate Biology| volume=130 |issue=2 |pages=115–128|doi=10.1111/j.1744-7410.2011.00226.x}}</ref> In addition, all sipunculans have numerous sensory nerve endings on the body wall, especially at the forward end of the introvert which is used for exploring the surrounding environment.<ref name=IZ/>

==Distribution and habitat==
[[File:Sipunculus nudus.jpeg|thumb|left|''[[Sipunculus nudus]]'']]
All sipunculid worms are marine and [[Benthos|benthic]]; they are found throughout the world's oceans including polar waters, equatorial waters and the [[abyssal zone]], but the majority of species occur in shallow water, where they are relatively common. They inhabit a range of habitats including burrowing in sand, mud, clay and gravel, hiding under stones, in rock crevices, in hollow coral heads, in wood, in empty [[seashell]]s and inside the bones of dead [[whale]]s. Some hide in kelp [[holdfast (biology)|holdfast]]s, under tangles of [[eelgrass]], inside [[sponge]]s and in the empty tubes of other organisms, and some live among [[fouling]] organisms on man-made structures.<ref name=Pancucci/><ref name=Walls/> Some bore into solid rocks to make a shelter for themselves.<ref name="Nguyen Thi Thu"/>

They are common below the surface of the sediment on [[tidal flat]]s. These worms may stay submerged in the sea bed for between 10 and 18 hours a day. They are sensitive to low salinities, and thus not commonly found near estuaries.<ref name= "Nguyen Thi Thu"/> They can also be abundant in [[coralline rock]], and in Hawaii, up to seven hundred individuals have been found per square metre in burrows in the rock.<ref name=Ruppert/>

==Reproduction==
Both asexual and sexual reproduction can be found in sipunculans, although asexual reproduction is uncommon and has only been observed in ''[[Aspidosiphon elegans]]'' and ''[[Sipunculus robustus]]''. These reproduce asexually through transverse [[fission (biology)|fission]], followed by [[Regeneration (biology)|regeneration]] of vital body components, with ''S. robustus'' also reproducing by [[budding]].<ref name=Rice>{{cite web |url=https://repository.si.edu/bitstream/handle/10088/4491/sms_rice_10.pdf |title=Sipuncula |author1=Rice, Mary E. |author2=Pilger, John F. |year=1988 |access-date=6 February 2019 |archive-url=https://web.archive.org/web/20170505215400/https://repository.si.edu/bitstream/handle/10088/4491/sms_rice_10.pdf |archive-date=5 May 2017 |url-status=dead }}</ref> One species of sipunculan, ''[[Themiste lageniformis]]'', has been recorded as reproducing [[Parthenogenesis|parthenogenetically]]; eggs produced in the absence of sperm developed through the normal stages.<ref name=Rice/>

Most sipunculan species are [[dioecious]]. Their [[gamete]]s are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment, where fertilisation takes place.<ref name=IZ/> In at least one species, ''[[Themiste pyroides]]'', swarming behaviour occurs with adults creating compact masses among rocks immediately before spawning.<ref name=Walls>{{cite book |title=Encyclopedia of Marine Invertebrates |last1=Walls |first1=Jerry G. |year=1982 |publisher=TFH Publications |isbn=978-0-86622-141-2 |pages=[https://archive.org/details/encyclopediaofma0000unse_f6h1/page/255 255–262] |url=https://archive.org/details/encyclopediaofma0000unse_f6h1/page/255 }}</ref>

Although some species hatch directly into the adult form, many have a [[trochophore]] larva, which metamorphoses into the adult after anything from a day to a month, depending on species. In a few species, the trochophore does not develop directly into the adult, but into an intermediate ''pelagosphaera'' stage, that possesses a greatly enlarged metatroch (ciliated band).<ref>{{cite journal |author=Rice, M.E. |year=1976 |title=Larval development and metamorphosis in Sipuncula |journal=American Zoologist |volume=16 |issue=3 |pages=563–571 |doi=10.1093/icb/16.3.563 |doi-access=free }}</ref> [[Metamorphosis]] occurs only in the presence of suitable habitat conditions, and is triggered by the presence of adults.<ref>{{cite journal |author1=Pechenik, J.A. |author2=Rice, M.E. |year=1990 |title=Influence of delayed metamorphosis on postsettlement survival and growth in the sipunculan ''Apionsoma misakianum'' |journal=Invertebrate Biology |volume=120 |issue=1 |pages=50–57 |doi=10.1111/j.1744-7410.2001.tb00025.x }}</ref><ref>{{cite journal |author=Rice, M.E. |year=1986 |title=Factors influencing larval metamorphosis in ''Golfingia misakiana'' (Sipuncula) |journal=Bulletin of Marine Science |volume=39 |issue=2 |pages=362–375 |url=https://www.ingentaconnect.com/content/umrsmas/bullmar/1986/00000039/00000002/art00017 }}</ref>

==Behaviour==
Most sipunculans are deposit feeders employing a number of different methods to obtain their foods. Those living in burrows extend their tentacles over the surface of the sediment. Food particles get trapped in mucous secretions and the beating of cilia transport the particles to the mouth. Among those that burrow through the sand, the tentacles are replaced by fluted folds which scoop up sediment and food particles. Most of this material is swallowed but larger particles are discarded. Species dwelling in crevices are able to withdraw their introverts, blocking the crevice entrance with their thickened trunks and presumably ingesting any food they have snared at the same time. One species, ''[[Thysanocardia procera]]'' is thought to be [[Carnivore|carnivorous]], gaining entrance in some way to the interior of the sea mouse ''[[Aphrodita aculeata]]'' and sucking out its liquefied contents.<ref name=Walls/>

==Fossil record==
[[File:Lecthaylus gregarius 5.jpg|thumb|''Lecthaylus gregarius'', a sipunculan from the Silurian of Illinois.]]

Because of their soft-bodied structure, fossils of sipunculans are extremely rare, and are only known from a few genera.  ''[[Archaeogolfingia]]'' and ''[[Cambrosipunculus]]'' appear in the [[Cambrian]] [[Maotianshan Shales|Chengjiang biota]] in China. These fossils appear to belong to the [[crown group]],<ref name="Huang2004">{{Cite journal | last1 = Huang | first1 = D. -Y. | last2 = Chen | first2 = J. -Y. | last3 = Vannier | first3 = J. | last4 = Saiz Salinas | first4 = J. I. | title = Early Cambrian sipunculan worms from southwest China | doi = 10.1098/rspb.2004.2774 | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 271 | issue = 1549 | pages = 1671–6 | date = 22 August 2004 | pmid =  15306286| pmc = 1691784}}</ref><ref name="Eibye-Jacobsen2012">{{Cite journal | last1 = Eibye-Jacobsen | first1 = D. | last2 = Vinther | first2 = J. | doi = 10.1111/j.1439-0469.2011.00651.x | title = Reconstructing the ancestral annelid | journal = Journal of Zoological Systematics and Evolutionary Research | volume = 50 | pages = 85–87 | date = February 2012 }}</ref> and demonstrate that sipunculans have changed little (morphologically) since the early Cambrian, about 520 million years ago.<ref name="Huang2004" />

An unnamed sipunculid worm from the Cambrian period has been discovered in the [[Burgess Shale]] in [[Alberta, Canada]],<ref name="Caron2010">{{Cite journal| last1 = Caron | first1 = J. -B.| last2 = Gaines | first2 = R. R.| last3 = Mangano | first3 = M. G.| last4 = Streng | first4 = M.| last5 = Daley | first5 = A. C.| title = A new Burgess Shale-type assemblage from the "thin" Stephen Formation of the southern Canadian Rockies| journal = Geology| volume = 38| issue = 9| pages = 811–814| year = 2010| doi = 10.1130/G31080.1| bibcode = 2010Geo....38..811C| url = https://scholarship.claremont.edu/cgi/viewcontent.cgi?article=1104&context=pomona_fac_pub}}</ref> and ''[[Lecthaylus]]'' has been identified from the [[Granton Shrimp Bed]], near Edinburgh, Scotland, dating to the [[Silurian]] period.<ref name="Muir2007">{{Cite journal | last1 = Muir | first1 = L. A. | last2 = Botting | first2 = J. P. | doi = 10.1144/sjg43010051 | title = A Lower Carboniferous sipunculan from the Granton Shrimp Bed, Edinburgh | journal = Scottish Journal of Geology | volume = 43 | pages = 51–56 | year = 2007 | issue = 1 | bibcode = 2007ScJG...43...51M | s2cid = 131119101 }}</ref> [[Trace fossil]]s of burrows that may have been formed by sipunculans have been found from the [[Paleozoic]].<ref name="Huang2004" />

Some scientists once hypothesized a close relationship between sipunculans and the extinct [[hyolith]]s, [[Operculum (gastropod)|operculate]] shells from the [[Palaeozoic]] with which they share a helical gut; but this hypothesis has since been discounted.<ref>{{cite journal |author1=Moysiuk,Joseph |author2=Smith, Martin R. |author3=Caron, Jean-Bernard |year=2017 |title=Hyoliths are Palaeozoic lophophorates |journal=Nature |volume=541 |issue=7637 |pages=394–397 |doi=10.1038/nature20804 |pmid=28077871 |bibcode=2017Natur.541..394M |s2cid=4409157 |url=http://dro.dur.ac.uk/20195/1/20195.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://dro.dur.ac.uk/20195/1/20195.pdf |archive-date=2022-10-09 |url-status=live }}</ref>

== As food ==
Sipunculid worm jelly ([[:zh:土笋冻|土笋凍]]) is a delicacy in southeast [[China]], originally from [[Anhai]], near [[Quanzhou]].<ref>{{cite web |url=https://neveryetmelted.com/2009/12/16/peanut-worm-jelly-its-whats-for-dinner/ |title=Peanut worm jelly: It's what's for dinner |author=Dorman, Jeremy Alban |date=16 December 2009 |work=The Telegraph |publisher=Never yet melted |access-date=9 February 2019 |archive-url=https://web.archive.org/web/20190209180323/https://neveryetmelted.com/2009/12/16/peanut-worm-jelly-its-whats-for-dinner/ |archive-date=9 February 2019 |url-status=dead }}</ref> A sipunculid worm dish is also considered a delicacy in the islands of the [[Visayas]] region, [[Philippines]]. The muscle is first prepared by soaking it in spiced vinegar and then served with other ingredients as a dish similar to [[ceviche]]. It is a basic food for local fisherman and is sometimes seen in city restaurants as an appetizer. This style of food preparation is locally called [[kilawin]] or [[kinilaw]], and is also used for fish, [[conch]] and vegetables.<ref name=TTN/>

The worms, especially in dried form, are considered a delicacy in [[Vietnam]] as well, where they are caught on the coasts of Minh Chao island, in [[Vân Đồn District|Van Don District]].<ref name=TTN>{{cite web|url=https://tuoitrenews.vn/news/features/20180616/women-earn-a-living-digging-for-peanut-worms-in-northern-vietnam/46159.html|title=Women earn a living digging for peanut worms in northern Vietnam |date=16 June 2018| publisher=Tuoi Tre News |access-date=9 February 2019}}</ref> The relatively high market price of the worms have made them a significant source of income for the local population of fishermen families.<ref name="Nguyen Thi Thu">Nguyen, Thi Thu Ha, et al. "The distribution of peanut-worm (''Sipunculus nudus'') in relation with geo-environmental characteristics." (2007).</ref>

<gallery widths="200px" heights="200px">
File:Sipunculid worm jelly.jpg|A plate of Sipunculid worm jelly.
File:Kilaw.jpg|This sipunculid worm dish is made by adding vinegar and local spices. Taken in [[Cebu]], Philippines.
</gallery>

== References ==
{{Reflist|30em}}

==External links==
{{Commons}}
* [http://www.ucmp.berkeley.edu/sipuncula/sipuncula.html Introduction to the Sipuncula, by UCMP]
* [http://tolweb.org/Sipuncula/2487 Sipuncula in "Tree of Life web project"]
* {{Cite EB1911|wstitle=Sipunculoidea|short=x}}

{{Taxonbar|from=Q205712}}
{{Authority control}}

[[Category:Sipunculans| ]]
[[Category:Spiralian classes]]
[[Category:Extant Cambrian first appearances]]
[[Category:Taxa named by Constantine Samuel Rafinesque]]