Editing Hemichordate (section)

== 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>