Editing Tusk shell (section)

=== Vascular system ===
The scaphopod vascular system is rudimentary lacking heart [[Atrium (heart)|auricles]] as well as corresponding [[Ctenidium (mollusc)|ctenidia]] (gills) and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. The [[heart]], a characteristic feature of all other groups of mollusca, has been considered totally lost or reduced to a thin fold of the [[pericardium]]; however, according to more recent studies, the muscular, regularly beating [[perianal]] blood sinus is [[Homology (biology)|homologous]] to the [[Ventricle (heart)|ventricle]] and is therefore considered the scaphopod heart.<ref>{{cite book |first=Patrick D. |last=Reynolds |contribution=Scaphopoda: The Tusk Shells |editor1-first=Charles F. |editor1-last=Sturm |editor2-first=Timothy A. |editor2-last=Pearce |editor3-first=Ángel |editor3-last=Valdés |title=The Mollusks: A guide to their study, collection, and preservation |place=Boca Ratón, FL |publisher=Universal Publishers |year=2006 |pages=229–238, esp. 231}}</ref>

Metabolic waste is excreted through a pair of [[nephridium|nephridia]] close to the anus.  The tusk shells appear to be the only extant molluscs which completely lack the otherwise standard molluscan [[reno-pericardial aperture]]s.  Furthermore, they also appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water (through two "water pores" located near the nephridial openings).  These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.<ref name=KhannaYadav2004>{{cite book |author1=Khanna, D.R. |author2=Yadav, P.R. |title=Biology of Mollusca |url=https://books.google.com/books?id=FlGw93oZocoC&pg=PA193 |date=1 January 2004 |publisher=Discovery |isbn=978-81-7141-898-5|page=198}}</ref>