The blood vascular system of Balanoglossus is the open or lacunar type.
It consists of:
1. Colourless blood,
2. Central sinus and a heart vesicle,
3. Distributing vessels or arteries and sinuses, and
4. Collecting vessels or veins.
The blood is colourless fluid containing few white corpuscles which are possibly detached endothelial cells. The respiratory pigment is probably absent.
2. Central Sinus and Heart Vesicle:
The central sinus is a small elongated non- contractile sinus located in the proboscis just above the buccal diverticulum. Just above it is a closed triangular cardiac sac or heart vesicle. Its ventral wall is muscular and contracts rhythmically. The central sinus receives blood from collecting vessels that open into its posterior end. Anteriorly, it pumps blood into several afferent vessels which form a plexus in the glomerulus lying in front of it. In glomerulus the blood gets rid of excretory wastes.
3. Arteries (Distributing Vessels):
Blood from the glomerulus is carried away by four arteries. Of these two arteries, a mid- dorsal proboscis artery and a mid- ventral proboscis artery, supply the proboscis. The other two, efferent glomerular arteries, run backward along the two sides of buccal diverticulum, encircle the buccal tube as peribranchial vessels (which are actually of the nature of plexuses) and unite in a single longitudinal ventral vessel that runs up to the posterior end of the body through the ventral mesentery.
The ventral vessel, on its way, gives out a ventral collar vessel to the collar, a ring vessel to the collar- trunk septum and an afferent branchial artery to each gill-septum in which it bifurcates to supply two adjacent tongue bars. All these branches break up into a system of sinuses in their respective structures. All along its length, the ventral vessel also supplies the body wall and gut wall by an elaborate network of sinuses. The ventral vessel has muscular contractile walls and the blood flows backwards in it.
4. Veins (Collecting Vessels):
Blood from body wall, gut wall and branchial apparatus (efferent branchial vessels) is collected by a single median dorsal vessel which runs through the dorsal mesentery, from posterior end up to the collar. It has muscular and contractile walls and the blood flows forward in it. At the anterior of collar, the dorsal vessel dilates a little to form a venous sinus. The venous sinus receives a lateral proboscis vein from each side of proboscis and then opens into the central sinus.