Editing Nemertea (section)

=== Nervous-system and senses ===

[[File:Amphiporus ochraceus brain.jpg|245px|Brain and neural cords of hoplonemertean ''Amphiporus ochraceus''. Several clusters of dark eyespots and the opening of one cerebral organ are also visible.|thumb]]

The [[central nervous system|central nervous-system]] consists of a [[brain]] and paired [[ventral nerve cord]]s that connect to the brain and run along the length of the body. The [[brain]] is a [[nerve ring|ring]] of four [[ganglia]], masses of nerve cells, positioned round the rhynchocoel near its front end<ref name="RFB2004NervousSenses" /> – while the brains of most [[protostome]] invertebrates encircle the foregut.<ref name="Maslakova2010Invention">{{cite journal|last=Maslakova|first=Svetlana A.|date=July 2010|title=The invention of the pilidium larva in an otherwise perfectly good spiralian phylum Nemertea|journal=[[Integrative and Comparative Biology]]|volume=50|issue=5|pages=734–743|doi=10.1093/icb/icq096|pmid=21558236|doi-access=free}}</ref> Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord.<ref name="RFB2004NervousSenses" /> In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers.<ref name=IZ/> The central nervous-system is often red or pink because it contains [[hemoglobin]]. This stores [[oxygen]] for peak activity or when the animal experiences [[Hypoxia (medical)|anoxia]], for example while [[burrowing]] in oxygen-free [[sediment]]s.<ref name="RFB2004NervousSenses">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | page=[https://archive.org/details/isbn_9780030259821/page/276 276] | url=https://archive.org/details/isbn_9780030259821/page/276 }}</ref>

Some species have paired [[cerebrum|cerebral]] organs, sacs whose only openings are to the outside. Others species have unpaired evertible organs on the front of their heads. Some have slits along the side of the head or grooves obliquely across the head, and these may be associated with paired cerebral organs. All of these are thought to be [[chemoreceptor]]s, and the cerebral organs may also aiding [[osmoregulation]]. Small pits in the epidermis appear to be sensors.<ref name="RFB2004NervousSenses" /> On their head, some species have a number of pigment-cup [[ocelli]],<ref name="RFB2004NervousSenses" /> which can detect light but not form an image.<ref>{{cite book|last1=Russell|first1=Peter J. |last2=Wolfe|first2=Stephen L. |last3=Hertz|first3=Paul E. |author4=Cecie Starr|title=Biology: the dynamic science|volume=3|year=2008|publisher=Cengage Learning |isbn=978-0-495-01034-0|pages=894–895|chapter=Photoreceptors and vision | chapter-url=https://books.google.com/books?id=VqkrMPg6E7EC&q=Russell,+Peter+J.;+Wolfe,+Stephen+L.;+Hertz,+Paul+E.;+Cecie+Starr+pigment-cup+ocelli&pg=PA894 | access-date=31 Jan 2011}}</ref> Most nemerteans have two to six ocelli, although some have hundreds.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 252–262|isbn= 978-0-03-056747-6}}</ref> A few tiny species that live between grains of sand have [[statocyst]]s,<ref name="RFB2004NervousSenses" /> which sense balance.<ref>{{cite book|last1=Russell|first1=Peter J. |last2=Wolfe|first2=Stephen L. |last3=Hertz|first3=Paul E. |author4=Cecie Starr|title=Biology: the dynamic science|volume=3|year=2008|publisher=Cengage Learning |isbn=978-0-495-01034-0|page=889|chapter=Photoreceptors and vision | chapter-url=https://books.google.com/books?id=VqkrMPg6E7EC&q=Russell,+Peter+J.;+Wolfe,+Stephen+L.;+Hertz,+Paul+E.;+Cecie+Starr+pigment-cup+ocelli&pg=PA894 | access-date=31 Jan 2011}}</ref>

''[[Paranemertes peregrina]]'', which feeds on polychaetes, can follow the prey's trails of mucus, and find its burrow by backtracking along its own trail of mucus.<ref name="RFB2004NutritionDigestive" />