Editing Echiura (section)

==Anatomy==
Spoon worms vary in size from the giant ''[[Ikeda taenioides]]'', nearly {{convert|2|m|ft|0|abbr=on}} long with its proboscis extended, to the minute ''[[Lissomyema]]'', measuring just {{convert|1|cm|in|1|abbr=on}}.<ref name=Ruppert/> 
Their bodies are generally cylindrical with two wider regions separated by a narrower region. There is a large extendible, scoop-shaped [[proboscis]] in front of the mouth which gives the animals their common name. This proboscis resembles that of [[Sipuncula|peanut worms]] but it cannot be retracted into the body. It houses a brain and may be [[Homology (biology)|homologous]] to the [[prostomium]] of other annelids.<ref name=IZ>{{cite book |author=Barnes, Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |location=Philadelphia, PA|pages=870–873|isbn=0-03-056747-5}}</ref> The proboscis has rolled-in margins and a groove on the ventral surface. The distal end is sometimes forked. The proboscis can be very long; in the case of the Japanese species ''[[Ikeda taenioides]]'', the proboscis can be {{convert|150|cm}} long while the body is only {{convert|40|cm}}. Even smaller species like ''[[Bonellia (annelid)|Bonellia]]'' can have a proboscis a metre (yard) long. The proboscis is used primarily for feeding. Respiration takes place through the proboscis and the body wall, with some larger species also using [[cloaca]]l irrigation. In this process, water is pumped into and out of the rear end of the gut through the anus.<ref name=Ruppert/><ref>{{cite web |url=http://www.reefs.org/library/aquarium_net/0497/0497_4.html |title=Part 6: Phylum Sipuncula and Phylum Annelida |author=Toonen, Rob |year=2012 |work=Reefkeeper's Guide to Invertebrate Zoology |access-date=10 November 2012 |archive-url=https://web.archive.org/web/20120826014338/http://www.reefs.org/library/aquarium_net/0497/0497_4.html |archive-date=26 August 2012 |url-status=dead }}</ref>

Compared with other annelids, echiurans have relatively few [[seta]]e (bristles). In most species, there are just two, located on the underside of the body just behind the proboscis, and often hooked. In others, such as ''Echiurus'', there are also further setae near the posterior end of the animal. Unlike most annelids, adult echiurans have no trace of segmentation.<ref name=IZ/> Most echiurans are a dull grey or brown but a few species are more brightly coloured, such as the translucent green ''[[Listriolobus pelodes]]''.<ref name=Walls>{{cite book |title=Encyclopedia of Marine Invertebrates |last=Walls |first=Jerry G. |year=1982 |publisher=TFH Publications |isbn=0-86622-141-7 |pages=[https://archive.org/details/encyclopediaofma0000unse_f6h1/page/262 262&ndash;267] |url=https://archive.org/details/encyclopediaofma0000unse_f6h1/page/262 }}</ref>

The body wall is muscular. It surrounds a large [[coelom]] which leads to a long looped intestine with an anus at the rear tip of the body.<ref name=II>{{cite book |title=Intertidal Invertebrates of the Central California Coast |last=Felty Light |first=Sol |year=1954|page=108 |publisher=University of California Press |url=https://books.google.com/books?id=2Qnod-98-q8C&q=scutum+tergum&pg=PA129 |access-date=30 November 2011 |isbn=9780520007505 }}</ref> The intestine is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched [[diverticulum|diverticula]] are connected to the [[rectum]]. These are lined with numerous minute [[Cilium|ciliated]] funnels that open directly into the body cavity, and are presumed to be excretory organs.<ref name=IZ/> The proboscis has a small coelomic cavity separated from the main coelom by a [[septum]].<ref name=Ruppert/>

Echiurans do not have a distinct respiratory system, absorbing oxygen through the body wall of both the trunk and proboscis, and through the cloaca in ''Urechis''.<ref name=Ruppert/> Although some species lack a blood vascular system, where it is present, it resembles that of other annelids. The blood is essentially colourless, although some [[haemoglobin]]-containing cells are present in the [[Coelom#Coelomic fluid|coelomic fluid]] of the main body cavity. There can be anywhere from one to over a hundred [[metanephridium|metanephridia]] for excreting nitrogenous waste, which typically open near the anterior end of the animal.<ref name=IZ/> The nervous system consists of a brain near the base of the proboscis, and a ventral nerve cord running the length of the body. Aside from the absence of segmentation, this is a similar arrangement to that of other annelids. Echiurans do not have any eyes or other distinct sense organs,<ref name=IZ/> but the proboscis is presumed to have a tactile sensory function.<ref name=Walls/>