The circulatory system of the earthworm is closed type as the blood flows in fine closed blood vessels. It includes blood, blood vessels, heart and anterior loops and blood glands.
1. Blood- A coloured respiratory pigment haemoglobin is found to be present in the plasma. Due to this, the colour of the plasma is red. In the blood, only one type of blood corpuscles, the leucocytes are present in the blood of the earthworm.
2. Blood vessels- the blood vessels found in earthworm are as follows:
Dorsal blood vessel- These extend from one end of the body to the other. The blood flows in this vessel from behind to forward on the dorsal side of the alimentary canal. The dorsal blood vessel has valves which prevent the backward flow of blood.
Ventral blood vessel- These extend from the one end to the other end of the body. It does not have any valves and flow of the blood is from the anterior to the posterior end of the body. It is the major distributing vessel.
Sub-neural blood vessel- It runs from the posterior end of the body up to the 14th segment in front. It collects blood from the body wall and nerve cord. This blood is then sent to the dorsal blood vessel through commissural vessels.
Lateral oesophageal vessel- These are the paired blood vessels lying one on either ventrolateral side of the alimentary canal between the body wall and the alimentary canal in the first 14th segment.
Supra-oesophageal blood vessel- It is a single vessel which lies on the dorsal side of the alimentary canal between the 9th and 13th segment. It receives blood from the lateral oesophageal through two pairs of anterior loops and pours into two pairs of latero-oesophageal hearts present in the 12th and 13th segments.
Hearts and anterior loops- In earthworm, four pairs of tubular hearts which are provided with valves. The anterior two pairs of the heart are known as the lateral heart and they lie in the 7th and 9th segment. They receive blood from the dorsal blood vessel and convey it to the ventral blood vessel. The posterior two pairs of hearts are situated in the 12th and 13th segments. These carry blood from the dorsal blood vessel and supra oesophageal vessel to ventral blood vessel. Two pairs of anterior loops are present in 10th and 11th segments.