Editing Sipuncula (section)

==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/>