Editing Hemichordate

{{Short description|Phylum of marine deuterostome animals}}
{{Automatic taxobox
| fossil_range = [[Miaolingian]]–[[Holocene|Present]] {{fossil range|506.5|0|earliest=535|}} 
(Possible [[Fortunian]] record)<ref>Maletz, J. (2024). "The evolutionary origins of the Hemichordata (Enteropneusta & Pterobranchia) - A review based on fossil evidence and interpretations". Bulletin of Geosciences. 99 (2): 127–147. doi:10.3140/bull.geosci.1899.</ref>
| image = Enteropneusta USNM 1431735, MBIO32531, BMOO 08254 - Specimen Image.jpg
| image_caption = [[Acorn worm]] (Enteropneusta)
| image2 = Cephalodiscus nigrescens.jpg
| image2_caption = ''[[Cephalodiscus nigrescens]]'' (Pterobranchia)
| display_parents = 6
| taxon = Hemichordata
| authority = [[William Bateson|Bateson]], 1885
| subdivision_ranks = Classes
| subdivision = * [[Planctosphaeroidea]]
* [[Enteropneusta]]
* [[Pterobranchia]]
}}

'''Hemichordata''' ({{IPAc-en|ˌ|h|ɛ|m|ɪ|k|ɔr|ˈ|d|eɪ|t|ə}} {{respell|HEM|ih|kor|DAY|tə}}) is a [[phylum]] which consists of [[triploblastic]], [[enterocoelomate|eucoelomate]], and [[Symmetry in biology#Bilatetal symmetry|bilaterally symmetrical]] [[marine life|marine]] [[deuterostome]] [[animal]]s, generally considered the sister group of the [[echinoderm]]s. They appear in the Lower or Middle [[Cambrian]] and include two main classes: [[Enteropneusta]] (acorn worms), and [[Pterobranchia]]. A third class, Planctosphaeroidea, is known only from the larva of a single species, ''[[Planctosphaera pelagica]]''. The class [[Graptolithina]], formerly considered extinct,<ref>{{cite journal|last=Sato|first=Atsuko|author2=Rickards RB |author3=Holland PWH |title=The origins of graptolites and other pterobranchs: a journey from 'Polyzoa'|journal=Lethaia|date=December 2008|volume=41|pages=303–316|doi=10.1111/j.1502-3931.2008.00123.x|issue=4|bibcode=2008Letha..41..303S }}</ref> is now placed within the pterobranchs, represented by a single living genus ''[[Rhabdopleura]]''.

Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest [[Secretion|secreted]] tubes)<ref name="Caron2013">{{Cite journal | last1 = Caron | first1 = J. B. | last2 = Conway Morris | first2 = S. | last3 = Cameron | first3 = C. B. | doi = 10.1038/nature12017 | title = Tubicolous enteropneusts from the Cambrian period | journal = Nature | volume = 495 | issue = 7442 | pages = 503–506 | year = 2013 | pmid =  23485974| bibcode = 2013Natur.495..503C | s2cid = 205233252 }}</ref> and are deposit feeders, but some species are pharyngeal [[filter feeders]], while the family  are free living [[detritivore]]s. Many are well known for their production and accumulation of various [[halogenated]] [[phenols]] and [[pyrroles]].<ref>{{cite journal|last=Giray|first=Cem|author2=G.M. King|title=Predator deterrence and 2,4-dibromophenol conservation by the enteropneusts, Saccoglossus bromophenolosus and Protoglossus graveolens|journal=Marine Ecology Progress Series|year=1997|volume=159|pages=229–238|doi=10.3354/meps159229|bibcode=1997MEPS..159..229G|doi-access=free}}</ref> [[Pterobranchs]] are filter-feeders, mostly colonial, living in a [[collagenous]] tubular structure called a [[coenecium]].<ref>{{cite definition coenecium (plural coenecia)
(biology) A branching tubular network inhabited by colonies of pterobranches journal|last=Sato|first=Atsuko|author2=Bishop JDD |author3=Holland PWH |title=Developmental biology of pterobranch hemichordates: history and perspectives|journal=Genesis|year=2008|volume=46|pages=587–91|doi=10.1002/dvg.20395|pmid=18798243|issue=11|s2cid=37286764 |doi-access=}}</ref>

The discovery of the [[stem group]] hemichordate ''[[Gyaltsenglossus]]'' shows that early hemichordates combined aspects of the two morphologically disparate classes.<ref name=ROM>{{cite magazine|last1=ROM |first1=Karma |title=A Cambrian Puzzle: An Ancient, Fossilized Marine Worm Crawls into the Light |magazine=ROM Magazine |publisher=Royal Ontario Museum |location=Toronto |date=Fall 2020}}</ref>

== Anatomy ==
The body plan of hemichordates is characterized by a muscular organization. The [[Anatomical terms of location|anteroposterior axis]] is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome.

The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and [[cilia|ciliated]] organ used in locomotion and in the collection and transport of food particles. The mouth is located between the proboscis and the collar. The trunk is the longest part of the animal. It contains the pharynx, which is perforated with gill slits (or pharyngeal slits), the oesophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile members of the acorn worm family [[Harrimaniidae]].<ref>{{cite journal | pmc= 5049775 | pmid=27701429 | doi=10.1371/journal.pone.0162564 | volume=11 | issue=10 | title=The Global Diversity of Hemichordata | journal=PLOS ONE | page=e0162564 | last1 = Tassia | first1 = MG | last2 = Cannon | first2 = JT | last3 = Konikoff | first3 = CE | last4 = Shenkar | first4 = N | last5 = Halanych | first5 = KM | author6-link=Billie J. Swalla | last6 = Swalla | first6 = BJ| bibcode=2016PLoSO..1162564T | year=2016 | doi-access=free }}</ref>

[[File:Journal.pbio.0040291.g001.svg|thumb|360px|left|{{center|Anatomy of ''Saccoglossus kowalevskii''<ref name=Lowe2006>{{cite journal | last1 = Lowe | first1 = C J | last2 = Terasaki | first2 = M | last3 = Wu | first3 = M | last4 = Freeman Jr | first4 = R M | last5 = Runft | first5 = L | last6 = Kwan | first6 = K | last7 = Gerhart | first7 = J | date = 22 August 2006 | title = Dorsoventral patterning in hemichordates: insights into early chordate evolution | journal = PLOS Biology | volume = 4 | issue = 9| page = e291 | doi = 10.1371/journal.pbio.0040291 | pmid = 16933975 | pmc = 1551926 | doi-access = free }}</ref>}}]]

The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium. The mesosome extends into one pair (in the genus ''Rhabdopleura'') or several pairs (in the genus ''Cephalodiscus'') of tentaculated arms used in filter feeding. The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of the colony, produced by asexual budding. In the genus ''Cephalodiscus'', asexually produced individuals stay attached to the contractile stalk of the parent individual until completing their development. In the genus ''[[Rhabdopleura]]'', zooids are permanently connected to the rest of the colony via a common [[stolon]] system.

Some species [[biomineralization|biomineralize]] in [[calcium carbonate]].<ref name="Cameron2012">{{Cite journal | last1 = Cameron | first1 = C. B. | last2 = Bishop | first2 = C. D. | doi = 10.1098/rspb.2012.0335 | title = Biomineral ultrastructure, elemental constitution and genomic analysis of biomineralization-related proteins in hemichordates | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 279 | issue = 1740 | pages = 3041–3048 | year = 2012 | pmid =  22496191| pmc = 3385480}}</ref>

They have a [[diverticulum]] of the foregut called a [[stomochord]], previously thought to be related to the chordate [[notochord]], but this is most likely the result of [[convergent evolution]] rather than a [[homology (biology)|homology]]. 

=== Neural system ===

A hollow [[neural tube]] exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of [[chordate|chordata]] and the rest of the deuterostomes.<ref>{{cite journal|last=Nomakstainsky|first=M|display-authors=etal|journal=Current Biology|date=11 August 2009|volume=19|pages=1264–9|doi=10.1016/j.cub.2009.05.063|pmid=19559615|title=Centralization of the deuterostome nervous system predates chordates|issue=15|doi-access=free|bibcode=2009CBio...19.1264N}}</ref> Hemichordates have a [[nerve net]] and longitudinal nerves, but no brain.<ref>{{cite journal | doi=10.1016/j.cub.2015.08.001 | title=Neural nets | year=2015 | last1=Hejnol | first1=Andreas | last2=Rentzsch | first2=Fabian | journal=Current Biology | volume=25 | issue=18 | pages=R782–R786 | pmid=26394095 | s2cid=18806753 | doi-access=free | bibcode=2015CBio...25.R782H }}</ref><ref>{{cite book | url=https://books.google.com/books?id=tu6XwCCRixsC&dq=%22weblike+nervous+system+that+runs%22&pg=PT138 | title=The Brain: Big Bangs, Behaviors, and Beliefs | isbn=9780300183566 | last1=Desalle | first1=Rob | last2=Tattersall | first2=Ian | date=30 April 2012 | publisher=Yale University Press }}</ref>

The nervous system of adult [[enteropneust]]s consists of:<ref>https://oxfordre.com/neuroscience/display/10.1093/acrefore/9780190264086.001.0001/acrefore-9780190264086-e-204</ref>
* the basiepidermal nerve net
* the [[dorsal nerve cord]]
* the [[ventral nerve cord]]
* the prebranchial nerve ring

===Circulatory system===
{{Expand section|date=November 2023}}
Hemichordates have an [[open circulatory system]]. The heart vesicle is located dorsally within the proboscis complex, and does not contain any blood. Instead it moves the blood indirectly by pulsating against the dorsal blood vessel.<ref>[https://books.google.com/books?id=9IWaqAOGyt4C&dq=Enteropneusta+Pterobranchia+heart+proboscis&pg=PA345 Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth]</ref>

== Development ==
Together with the [[echinoderms]], the hemichordates form the [[Ambulacraria]], which are the closest extant phylogenetic relatives of [[chordates]]. Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly.<ref>{{cite book | pmid = 15575607 | volume = 74 | year = 2004 | pages = [https://archive.org/details/isbn_9780124802780/page/171 171–194] | last1 = Lowe | first1 = CJ | last2 = Tagawa | first2 = K | last3 = Humphreys | first3 = T | last4 = Kirschner | first4 = M | last5 = Gerhart | first5 = J | title = Development of Sea Urchins, Ascidians, and Other Invertebrate Deuterostomes: Experimental Approaches | chapter = Hemichordate Embryos: Procurement, Culture, and Basic Methods | doi = 10.1016/S0091-679X(04)74008-X | series = Methods in Cell Biology | isbn = 9780124802780 | url = https://archive.org/details/isbn_9780124802780/page/171 | editor1= Charles A. Ettensohn| editor2= Gregory A. Wray| editor3= Gary M. Wessel }}</ref> Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm.

The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm.<ref>{{cite journal | last1 = Tagawa | first1 = K. | last2 = Nishino | first2 = A | last3 = Humphreys | first3 = T | last4 = Satoh | first4 = N. | date = 1 January 1998 | title = The Spawning and Early Development of the Hawaiian Acorn worm (Hemichordate), Ptycodhera flava | journal = Zoological Science | volume = 15 | issue = 1| pages = 85–91 | doi = 10.2108/zsj.15.85 | pmid = 18429670 | hdl = 2433/57230 | s2cid = 36332878 | hdl-access = free }}</ref> The Pterobranch genus most extensively studied is ''Rhabdopleura'' from Plymouth, England and from Bermuda.<ref>{{cite journal|last=Stebbing|first=ARD|title=Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)|journal=Marine Biology|year=1970|volume=5|pages=205–212|doi=10.1007/BF00346908|issue=3|bibcode=1970MarBi...5..205S |s2cid=84014156}}</ref><ref>{{cite journal|last=Dilly|first=PN|title=The larva of Rhabdopleura compacta (Hemichordata)|journal=Marine Biology|date=January 1973|volume=18|issue=1 |pages=69–86|doi=10.1007/BF00347923|bibcode=1973MarBi..18...69D |s2cid=86563917}}</ref><ref>{{cite journal|last=Lester|first=SM|title=Settlement and metamorphosis of Rhabdopleura normani (Hemichordata: Pterobranchia)|journal=Acta Zoologica|date=June 1988|volume=69|pages=111–120|doi=10.1111/j.1463-6395.1988.tb00907.x|issue=2}}</ref><ref>{{cite journal|last=Lester|first=SM|title=Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleura normani|journal=Acta Zoologica|year=1986|volume=69|pages=95–109|doi=10.1111/j.1463-6395.1988.tb00906.x|issue=2}}</ref>

The following details the development of two popularly studied species of the hemichordata phylum ''Saccoglossus kowalevskii'' and ''Ptychodera flava''. ''Saccoglossus kowalevskii'' is a direct developer and ''Ptychodera flava'' is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.

[[File:Hemichordate development.jpg|thumb|320px|right|Schematic of embryonic cleavage and development in ''P. flava'' and ''S. kowalevskii'']]

=== ''Ptychodera flava'' ===
''P. flava’s'' early cleavage pattern is similar to that of ''S. kowalevskii''. The first and second cleavages from the single cell zygote of ''P. flava'' are equal cleavages, are [[orthogonal]] to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four [[blastomere]]s both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. ''P. flava'' has a 16 cell embryo with four vegetal micromeres, eight animal [[mesomere]]s and four larger macromeres. Further divisions occur until ''P. flava'' finishes the [[blastula]] stage and goes on to [[gastrulation]]. The animal mesomeres of ''P. flava'' go on to give rise to the larva’s [[ectoderm]], animal [[blastomere]]s also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues.<ref>{{cite journal | pmid = 11806633 | volume=3 | issue=6 | title=Deuterostome evolution: early development in the enteropneust hemichordate, ''Ptychodera flava'' | date=November–December 2001| pages=375–90 | doi=10.1046/j.1525-142x.2001.01051.x | journal=Evolution & Development | last1 = Henry | first1 = JQ | last2 = Tagawa | first2 = K | last3 = Martindale | first3 = MQ| s2cid=24071389 }}</ref> Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development ''P. flava'' blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage.<ref>{{cite journal | last1 = Colwin | first1 = A | last2 = Colwin | first2 = L | year = 1950 | title = The developmental capacities of separated early blastomeres of an enteropneust, ''Saccoglossus kowalevskii'' | journal = Journal of Experimental Zoology | volume = 155 | issue = 2| pages = 263–296| doi = 10.1002/jez.1401150204  | bibcode = 1950JEZ...115..263C }}</ref>

=== ''Saccoglossus kowalevskii'' ===
Eggs of ''S. kowalevskii'' are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm.<ref>{{cite journal | last1 = Colwin | first1 = A | last2 = Colwin | first2 = L | year = 1951 | title = Relationships between the egg and larva of Saccoglossus kowalevskii (Enteropneusta): axes and planes; general prospective significance of the early blastomeres | journal = Journal of Experimental Zoology | volume = 117 | issue = 1 | pages = 111–138 | doi=10.1002/jez.1401170107| bibcode = 1951JEZ...117..111C }}</ref><ref>{{cite journal |first1=Arthur L. |last1=Colwin |first2=Laura Hunter |last2=Colwin |date=May 1953 |title=The normal embryology of saccoglossus kowalevskii (enteropneusta) | journal = Journal of Morphology | volume = 92 |issue=3 | pages = 401–453 | doi = 10.1002/jmor.1050920302 |s2cid=85420179 }}</ref>

=== Genetic control of dorsal-ventral hemichordate patterning ===
Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on ''S. kowalevskii'', and like chordates ''S. kowalevskii'' has dorsalizing bmp-like factors such as ''bmp 2/4'', which is homologous to ''Drosophila''’s decapentaplegic dpp. The expression of ''bmp2/4'' begins at the onset of gastrulation on the ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post-anal tail. The bmp antagonist chordin is also expressed in the [[endoderm]] of gastrulating ''S. kowalevskii''. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in ''S. kowalevskii'', such as a netrin that groups with netrin gene class 1 and 2.<ref name=Lowe2006 /> Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but ''S. kowalevskii'' was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.

== Classification ==
{{See also|List of bilateral animal orders}}
[[File:DiplograptusCaneySprings.jpg|thumb|right|''Amplexograptus'', a [[graptolite]] hemichordate, from the [[Ordovician]] near [[Caney Springs, Tennessee|Caney Springs]], [[Tennessee]].]]

Hemichordata are divided into two classes: the [[Acorn worm|Enteropneusta]],<ref>{{Cite journal | last1 = Cameron | first1 = CB | last2 = Garey | first2 = JR | author3-link = Billie J. Swalla | last3 = Swalla | first3 = BJ | title = Evolution of the chordate body plan: new insights from phylogenetic analyses of deuterostome phyla | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 97 | issue = 9 | pages = 4469–74 | date = 25 April 2000 | pmid = 10781046 | pmc = 18258 |bibcode = 2000PNAS...97.4469C |doi = 10.1073/pnas.97.9.4469 | doi-access = free }}</ref> commonly called acorn worms, and the [[Pterobranchia]], which includes the [[graptolite]]s.<ref name="Rhabdopleura2020">{{cite journal |last1=Ramírez-Guerrero |first1=G. M. |last2=Kocot, K. M. |last3=Cameron |first3=C. B. |year=2020 |title=Zooid morphology and molecular phylogeny of the graptolite ''Rhabdopleura annulata'' (Hemichordata, Pterobranchia) from Heron Island, Australia |journal=Canadian Journal of Zoology |volume=98 |issue=12 |pages=844–849|doi=10.1139/cjz-2020-0049 |s2cid=228954598 }}</ref> A third class, [[Planctosphaeroidea]], is proposed based on a single species known only from larvae. The phylum contains about 120 living species.<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| 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| doi=10.11646/zootaxa.3148.1.3 }}</ref> Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic. It is possible that the extinct organism ''[[Etacystis]]'' is a member of the Hemichordata, either within or with close affinity to the Pterobranchia.<ref>{{cite journal |url=http://jpaleontol.geoscienceworld.org/content/50/6/1157.short |title=Etacystis communis, a Fossil of Uncertain Affinities from the Mazon Creek Fauna (Pennsylvanian of Illinois) |journal=[[Journal of Paleontology]] |volume=50 |date= November 1976 |pages=1157–1161}}</ref>

There are 130 described species of Hemichordata and many new species are being discovered, especially in the deep sea.<ref name=Tassia2016>{{cite journal | last1 = Tassia | first1 = MG | last2 = Cannon | first2 = JT | last3 = Konikoff | first3 = CE | last4 = Shenkar | first4 = N | last5 = Halanych | first5 = KM | author6-link = Billie J. Swalla | last6 = Swalla | first6 = BJ | year = 2016 | title = The Global Diversity of Hemichordata | journal = PLOS ONE | volume = 11 | issue = 10| page = e0162564 | doi = 10.1371/journal.pone.0162564 | pmid=27701429 | pmc=5049775| bibcode = 2016PLoSO..1162564T | doi-access = free }}</ref>

===Phylogeny===
A [[phylogenetic tree]] showing the position of the hemichordates is:

{{clade
|1={{clade
   |label1=[[Chordata]]
   |1={{clade
      |1=[[Cephalochordata]] [[File:Branchiostoma lanceolatum (Pallas, 1774).jpg|80 px]]
      |label2=[[Olfactores]]
      |2={{clade
         |1=[[Tunicata]] [[File:Tunicate komodo.jpg|60 px]]
         |2=[[Vertebrate|Vertebrata]]/[[Craniata]] [[File:Common carp (white background).jpg|60 px]] 
         }}
      }}
   |label2=[[Ambulacraria]]
   |2={{clade
      |1=[[Echinodermata]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
      |2='''Hemichordata''' [[File:Saccoglossus kowalevskii by Spengel 1893.png|50 px]]
      }}
   }}|label1=[[Deuterostomia]]|style=font-size:80%; line-height:80%}}

The internal relationships within the hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.<ref>{{Cite journal |last1=Tassia |first1=Michael G. |last2=Cannon |first2=Johanna T. |last3=Konikoff |first3=Charlotte E. |last4=Shenkar |first4=Noa |last5=Halanych |first5=Kenneth M. |author6-link=Billie J. Swalla |last6=Swalla |first6=Billie J. |date=2016-10-04 |title=The Global Diversity of Hemichordata |journal=PLOS ONE |volume=11 |issue=10 |pages=e0162564 |doi=10.1371/journal.pone.0162564 |pmid=27701429 |pmc=5049775|bibcode=2016PLoSO..1162564T |doi-access=free }}</ref><ref>{{Cite journal|last1=Halanych|first1=Kenneth M.|last2=Bernt|first2=Matthias|last3=Cannon|first3=Johanna T.|last4=Tassia|first4=Michael G.|last5=Kocot|first5=Kevin M.|last6=Li|first6=Yuanning|date=2019-01-01|title=Mitogenomics Reveals a Novel Genetic Code in Hemichordata|journal=Genome Biology and Evolution|language=en|volume=11|issue=1|pages=29–40|doi=10.1093/gbe/evy254|pmc=6319601|pmid=30476024}}</ref><ref name="Maletz">{{cite journal |last=Maletz |first=Jörg| title=The classification of the Pterobranchia (Cephalodiscida and Graptolithina) |journal=Bulletin of Geosciences |year=2014 |volume=89 |issue=3 |pages=477–540 |doi=10.3140/bull.geosci.1465 |issn=1214-1119|doi-access=free }}</ref>

{{clade
|label1='''Hemichordata'''|style=font-size:80%; line-height:80%
|1={{clade
  |label1=[[Enteropneusta]]
  |1={{clade
    |1=''[[Stereobalanus]]''
    |2={{clade
      |1=[[Harrimaniidae]] [[File:Saccoglossus.jpg|60 px]] 
      |2={{clade
        |1=[[Spengeliidae]]
        |label2=
        |2={{clade
          |1=[[Torquaratoridae]]
          |2=[[Ptychoderidae]] [[File:Saccoglossus kowalevskii by Spengel 1893.png|50 px]]
           }}
         }}
       }}
     }}
  |label2=[[Pterobranchia]]
  |2={{clade
    |1=[[Cephalodiscida]] [[File:Cephalodiscus dodecalophus McIntosh.png|50 px]]
    |label2=[[Graptolithina]]
    |2={{clade
      |1=[[Rhabdopleurida]] [[File:Rhabdopleura normani Sedgwick.png|30px]]
      |label2=†[[Eugraptolithina]]
      |2={{clade
        |1=†[[Dendroidea]] [[File:Estonian Museum of Natural History - Dictyonema.png|60px]]
        |2=†[[Graptoloidea]] [[File:Tetragraptus serra (Brongniart).jpg|60px]]
         }}
       }}
     }}
   }}
}}

== References ==
{{Reflist|32em}}

==Other references==
* {{cite journal | last1 = Cameron | first1 = C.B. | year = 2005 | title = A phylogeny of the hemichordates based on morphological characters | journal = Canadian Journal of Zoology | volume = 83 | issue = 1| pages = 196–2 | doi=10.1139/z04-190}}

== External links ==
{{Wikispecies|Hemichordata}}
{{Commons category|Hemichordata}}
{{EB1911 Poster|Hemichorda}}
* [http://mapageweb.umontreal.ca/cameroc/ Cameron, C. B. ''Evolution and classification of Hemichordata'']
* [http://faculty.washington.edu/bjswalla Dr. Billie J. Swalla on the WEB "Development and Evolution of Hemichordates"]
* [https://web.archive.org/web/20101024162013/http://gump.auburn.edu/halanych/lab/ Dr. Ken Halanych "Phylogeny and Evolution of Hemichordates"]
* [https://web.archive.org/web/20100212043258/http://www.lowelab.org/Lowe_Lab/Home.html Dr. Chris Lowe "Genomics and Development of Saccoglossus kowalevskii"]
* [http://tolweb.org/tree?group=Hemichordata&contgroup=Deuterostomia At the Tree of Life] {{Webarchive|url=https://web.archive.org/web/20081206100653/http://tolweb.org/tree?group=Hemichordata&contgroup=Deuterostomia |date=2008-12-06 }}

{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from=Q174301}}
{{Authority control}}

[[Category:Hemichordates| ]]
[[Category:Miaolingian first appearances]]
[[Category:Extant Cambrian first appearances]]
[[Category:Taxa named by William Bateson]]