Editing Heart (section)

===Invertebrates===
[[File:Structural organization of the heart of the mosquito Anopheles gambiae - image.ppat.v08.i11.g001.png|thumb|The tube-like heart (green) of the mosquito ''[[Anopheles gambiae]]'' extends horizontally across the body, interlinked with the diamond-shaped [[insect wing#Muscles|wing muscles]] (also green) and surrounded by [[pericardium|pericardial cells]] (red). Blue depicts [[cell nuclei]].]]
[[File:Arthropod body struct 01.png|thumb|left|Basic [[arthropod]] body structure – heart shown in red]]
[[Arthropod]]s and most [[mollusk]]s have an open circulatory system. In this system, deoxygenated blood collects around the heart in cavities ([[:wikt:sinus|sinuses]]). This blood slowly permeates the heart through many small one-way channels. The heart then pumps the blood into the [[hemocoel]], a cavity between the organs. The heart in arthropods is typically a muscular tube that runs the length of the body, under the back and from the base of the head. Instead of blood the circulatory fluid is [[haemolymph]] which carries the most commonly used [[respiratory pigment]], copper-based [[haemocyanin]] as the oxygen transporter. Haemoglobin is only used by a few arthropods.<ref name=SOLOMON2010>{{Cite book|url=https://books.google.com/books?id=qdQ8AAAAQBAJ|title=Biology|last1=Solomon|first1=Eldra|last2=Berg|first2=Linda|last3=Martin|first3=Diana W.|year=2010|page=939|publisher=Cengage Learning|isbn=978-1-133-17032-7|language=en|url-status=live|archive-url=https://web.archive.org/web/20161206054642/https://books.google.com/books?id=qdQ8AAAAQBAJ|archive-date=6 December 2016}}</ref>
[[File:Cephalopod systemic heart.svg|thumb|left|Schematic of [[cephalopod]] heart]]
In some other invertebrates such as [[earthworm]]s, the circulatory system is not used to transport oxygen and so is much reduced, having no veins or arteries and consisting of two connected tubes. Oxygen travels by diffusion and there are five small muscular vessels that connect these vessels that contract at the front of the animals that can be thought of as "hearts".<ref name=SOLOMON2010/>

[[Cephalopod|Squids and other cephalopods]] have two "gill hearts" also known as [[branchial heart]]s, and one "systemic heart".<ref>Schipp, R., von Boletzky, S., Jakobs, P. et al. "A congenital malformation of the systemic heart complex in Sepia officinalis L. (Cephalopoda)". Helgoländer Meeresunters. 52, 29–40 (1998). {{doi|10.1007/BF02908733}}</ref> The branchial hearts have two atria and one ventricle each, and pump to the [[gill]]s, whereas the systemic heart pumps to the body.<ref>{{cite web|title=Meet our animals|url=https://nationalzoo.si.edu/Animals/Invertebrates/facts/cephalopods/default.cfm|website=Smithsonian National Zoological Park|access-date=14 August 2016|url-status=dead|archive-url=https://web.archive.org/web/20160729081825/https://nationalzoo.si.edu/Animals/Invertebrates/Facts/cephalopods/default.cfm|archive-date=29 July 2016}}</ref><ref name="Prosser1991">{{cite book|vauthors=Ladd, Prosser C|title=Comparative Animal Physiology, Environmental and Metabolic Animal Physiology|url=https://books.google.com/books?id=7fQvbFlQBaQC&pg=PA537|year=1991|publisher=John Wiley & Sons|isbn=978-0-471-85767-9|pages=537–|url-status=live|archive-url=https://web.archive.org/web/20161206215610/https://books.google.com/books?id=7fQvbFlQBaQC&pg=PA537|archive-date=6 December 2016}}</ref>
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Only the [[chordates]] (including vertebrates) and the [[hemichordates]] have a central "heart", which is a vesicle formed from the thickening of the [[aorta]] and contracts to pump blood. This suggests a presence of it in the last [[deuterostome|common ancestor of these groups]] (may have been lost in the [[echinoderms]]).