Blood body wall,gut wall and branchial apparatus (efferent branchial vessels)s collected by a single median dorsal vessel which runs through the dorsal mesentery, from posterior end upto the collar. It has muscular and contractile walls and the blood flows in it forwards. At the anterior end of collar, the dorsal vessel dilates a little to form a venous sinus. The latter receives a lateral proboscis vein from each side of proboscis and then opens into the central sinus.
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