Editing Sipuncula (section)

===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>