normal circulation the blood is pumped forwards through the heart at systole, passing out of the heart via the excurrent ostia and, anteriorly, from the aorta.The dorsal diaphragm is usually convex above so that contraction of the alary muscles tends to flatten it. This flattening increases the volume of the pericardial sinus at the expense of the perivisceral sinus so that this causes a flow of blood up into the pericardial sinus, and then into the heart at diastole through the incurrent ostia.Blood percolates down to the perineural sinus where it is agitated by movements of the ventral diaphragm which assist the blood flow to the nerve cord and possibly produce a backward flow of blood.