It is open type and consists of the following parts:
(1) Blood:
Due to the absence of respiratory pigment blood is colourless. But it contains a few white corpuscles.
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(2) Central sinus and heart vesicle:
Just above the buccal diverticulum a small elongated non-contractile central sinus is situated. Just above it, is a closed triangular cardiac sac or heart vesicle.
The ventral wall of heart versicle is muscular, contracts rhythmically thereby producing pulsations in the central sinus and helps in blood circulation.
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On the posterior end central sinus receives blood from collecting vessels. Anteriorly it pumps its blood into several afferent vessels which form a plexus in the proboscis glands or glomerulus lying in front of it. In glomerulus the body gets rid of its excretory wastes.
(3) Distributing vessels or arteries:
From the glomerulus, the blood is distributed in different parts of the body through four arteries, a mid dorsal and a mid ventral artery supplies to the proboscis.
Rest of the two arteries, called efferent glomerular arteries make a plexus near the buccal diverticulum.
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They unite in a single longitudinal ventral vessel that runs up to the posterior end of the body along with the ventral mesentry.
On its way, the ventral vessel gives out a ventral collar vessel to the collar, a ring vessel to the collar trunk septum and an efferent branchial artery to each gill septum in which it bifurcates to supply two adjacent tongue bars.
All along its length the ventral vessel also supplies the body wall and gut wall by an elaborate network of sinuses. Ventral vessel has muscular contractile walls and the blood in it flows backward.
(4) Collecting Vessels or Veins:
Blood from body wall, gut wall and branchial apparatus is collected by a single median dorsal vessel which runs through the dorsal mesentery.
It has muscular and contractile wall and the blood forwards. At the anterior end of collar the dorsal vessel dilates a little to form a venous sinus. It receives a lateral proboscis vein from each side of proboscis and then opens into the central sinus.