Editing Onychophora (section)

===Circulation===
The body cavity is known as a "pseudocoel", or [[haemocoel]]. Unlike a true [[coelom]], a pseudocoel is not fully enclosed by a cell layer derived from the embryonic [[mesoderm]]. A coelom is, however, formed around the [[gonad]]s and the waste-eliminating [[nephridia]].<ref name=Boudreaux/> As the name ''haemocoel'' suggests, the body cavity is filled with a [[blood]]-like liquid in which all the organs are embedded; in this way, they can be easily supplied with [[nutrient]]s circulating in the blood. This liquid is colourless as it does not contain [[pigment]]s; for this reason, it serves only a limited role in [[oxygen]] transport.
Two different types of blood cells (or haemocytes) circulate in the fluid: [[Amoebocyte]]s and nephrocytes. The amoebocytes probably function in protection from [[bacteria]] and other foreign bodies; in some species, they also play a role in [[reproduction]]. Nephrocytes absorb [[toxin]]s or convert them into a form suitable for [[clearance (medicine)|elimination]] by the nephridia.{{citation needed|date=November 2014}}

The haemocoel is divided by a horizontal partition, the diaphragm,<!-- Do '''''not''''' link this to any article referring to any kind of Chordate diaphragm, in particular not the thoracic diaphragm. They have nothing to do with each other. --> into two parts: The [[pericardial sinus]] along the back and the perivisceral sinus along the belly. The former encloses the tube-like heart, and the latter, the other organs. The diaphragm is perforated in many places, enabling the exchange of fluids between the two cavities.{{citation needed|date=November 2014}} The heart itself is a tube of annular muscles consisting of [[epithelium|epithelial]] tissues, with two lateral openings ([[wikt:ostium|ostia]]) per segment. While it is not known whether the rear end is open or closed, from the front, it opens directly into the body cavity.

Since there are no blood vessels, apart from the fine vessels running between the muscle layers of the body wall and a pair of arteries that supply the antennae, this is referred to as an [[open circulation]].{{citation needed|date=November 2014}} The timing of the pumping procedure can be divided into two parts: [[Diastole]] and [[systole (medicine)|systole]]. During diastole, blood flows through the ostia from the pericardial sinus (the cavity containing the heart) into the heart. When the systole begins, the ostia close and the heart muscles contract inwards, reducing the volume of the heart. This pumps the blood from the front end of the heart into the perivisceral sinus containing the organs. In this way, the various organs are supplied with nutrients before the blood finally returns to the pericardial sinus via the perforations in the diaphragm. In addition to the pumping action of the heart, body movements also influence circulation.{{citation needed|date=November 2014}}