Editing Nemertea (section)

=== Respiration and circulatory system ===
Nemerteans lack specialized [[gills]], and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid [[Circulatory system|circulation]] rather than [[diffusion]] to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels,<ref name="RFB2004RespirationCirculatoryExcretion" /> while their [[Blood#Invertebrates|blood]] is contained in the main body cavity.<ref name="Moore" /> The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluid [[skeleton]] in everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation.<ref name="RFB2004RespirationCirculatoryExcretion">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | pages=[https://archive.org/details/isbn_9780030259821/page/275 275–276] | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | url=https://archive.org/details/isbn_9780030259821/page/275 }}</ref> The circulatory vessels are a system of coeloms.<ref name="Pérez-PomaresEtc2009Card">{{cite journal
 |last       = Pérez-Pomares
 |first      = José M.
 |author2    = Juan M. González-Rosa
 |author3    = Ramón Muñoz-Chápuli
 |title      = Building the vertebrate heart - an evolutionary approach to cardiac development
 |journal    = The International Journal of Developmental Biology
 |year       = 2009
 |volume     = 53
 |issue      = 8–9–10
 |pages      = 1427–1443 [1430]
 |doi        = 10.1387/ijdb.072409jp
 |pmid      = 19247975
 |doi-access= free
 }}</ref>

In the simplest type of circulatory system, two lateral vessels are joined at the ends to form a loop. However, many species have additional long-wise and cross-wise vessels. There is no heart nor pumping vessels,<ref name="Anderson1998InvertebratePhyla">{{cite book|last1=Anderson|first1=D.T.|editor=D.T. Anderson|title=Invertebrate Zoology|edition=1|year=1998|publisher=Oxford University Press Australia|isbn=978-0-19-553941-7|page=4|chapter=The invertebrate phyla}}</ref> and the flow of fluid depends on contraction of both the vessels and the body wall's muscles. In some species, circulation is intermittent, and fluid ebbs and flows in the long-wise vessels.<ref name="RFB2004RespirationCirculatoryExcretion" /> The fluid in the vessels is usually colorless, but in some species it contains cells that are yellow, orange, green or red. The red type contain [[hemoglobin]] and carry oxygen, but the function of the other pigments is unknown.<ref name="RFB2004RespirationCirculatoryExcretion" />