Earthworm circulatory system

Circulatory System

The circulatory system is of the closed type. It is formed of three components, namely blood, blood vessels and hearts. 1. Blood It is reddish in colour. It is formed of plasma and corpuscles. The corpuscles are amoeboid, colourless and nucleated. The plasma contains a red pigment called haemoglobin. 2. Blood Vessels Earthworm has the following blood vessels.

1. Dorsal vessel

 2. Ventral vessel

3. Subneural vessel.

4. Supra oesophageal vessel

5. Lateral oesophageal vessel

6. Anterior loops

7. Ring vessels

8. Ventro-tegumentary vessels

9. Commissural vessels

10.Dorso intestinal vessels

11.Ventro intestinal vessels

12.Intestinal plexuses

1.Dorsal Vessel Dorsal vessel is a longitudinal blood ves-sel lying above the alimentary canal. It extends through the length of the body. It is the largest blood vessel. It is contractile. The blood flows forwards in the dorsal vessel. It has a pair of valves in each segment. It prevents the backward flow of blood. It is thick walled. The anterior part of the dorsal vessel, upto the 13th segment, functions as the dis-tributing vessel. It supplies blood to the ven-tral vessel through 4 pairs of hearts. At the anterior end, the dorsal vessel divides into three branches supplying blood to the phar-ynx and buccal chamber. It also supplies blood to the oesophagus, gizzard and pharyngeal nephridia by paired vessel. Behind the 13th segment the dorsal vessel functions as the collecting vessel. It collects blood from the subneural vessel through a pair of commissural vessels and from the intestine through two pairs of dorso intestinal vessels in each segment.

2.Ventral Vessel It extends anteroposteriorly below the ali-mentary canal. The walls are thin. It is nonvessel  contractile. The blood flows backwards. It has no valves. It is a distributing vessel. The ventral vessel supplies blood to the body wall, intestine and nephridia. In each segment, a pair of ventro tegu-mentary supplies the body wall. Behind the 13th segment each ventro-tegumentary gives a branch called septonephridial branch sup-pling the septal nephridia. Behind the 13th segment, a median vessel called ventro intestinal arises in each seg-ment and supplied blood to the intestine

3. Subneural Vessel .It lies below the ventral nerve cord in the median line. It starts from the 14th segment and extends upto the posterior end. Anteriorly it is formed by union of two lateral oesophageal vessels. Blood flows backwards. It is a collect-ing vessel. It collects blood from the nerve cord and ventral part of the body wall by a pair of vessels in each segment. A pair of commissural vessels in each segment sends blood to the dorsal vessel

4. Supra Oesophageal Vessel It is a median vessel lying on the stomach from the 9th to the 13th segment. It collects blood from the gizzard and stomach. It also receives blood from the lateral oesophageal vessels by two pairs of anterior loops in the 101h and 11th segments. It sends its blood to the posterior two pairs of hearts.

5.Lateral Oesophageal Vessels These are two lateral vessels lying on ei-ther side of the gut extending from the ante-rior end to the 13th segment. Behind the 13th segment they unite to form a median subneu-ral vessel. They collect blood from the body wall, septa nephridia and reproductive organs through a pair of ventro-tegumentary vessels in each segment. In the 10th 11 th segments, they are con-nected to the supraoesophageal vessel through a pair of anterior loops In the supraoesophageal vessel, the blood flows backwards.

6. Anterior Loops Anterior loops are two pairs of blood vessels connecting lateral oesophageal vessels to the supraoesophageal vessel. The blood flows upwards in the vessels. They are located in the 10th and 11th segments.

7. Ring Vessels These vessels are present in the wall of the gut. They connect the lateral oesophageal vessels to the supraoesophageal vessel. There are 12 vessels in each segment.

8.Ventro-tegumentary Vessels Ventro-tegumentary vessels are paired vessels arising from the ventral vessel. They supply blood to the body wall, septa, nephridia and reproductive organ. Behind the 13th seg-ment each ventro-tegumentary vessel supplies a vessel called septonephridial vessel to the septal nephridium.

9. Commissural Vessels Commissural vessels are paired vessels supplying blood from the subneural vessel to  the dorsal vessel. There is one pair in each segment from the 13th segment. Each commissural vessel gives a branch septointestinal vessel to the intestinal plexus.

10. Dorso Intestinal Vessels Dorso-intestinal vessels are paired ves-sels collecting blood from the intestine to the dorsal vessel. There are two pairs of such ves-sels in each segment.

11. Ventro Intestinal Vessels :In each segment, the intestine receives blood through a medium vessel from the ven-tral vessel called ventro intestinal vessel.

12.1ntestinal Plexus :The intestinal wall contains two networks of blood vessels called intestinal plexus. They are external plexus and internal plexus. The external plexus is located on the surface of the intestine. It receives blood from the ventral vessel through the ventrointestinal vessel and septal intestinal vessel. The internal plexus is situated inside the intestinal wall. It receives blood from the ex-ternal plexus and transports it to the dorsal ves-sel through the dorso intestinal vessel.

III. Hearts :Earthworm has 4 pairs of hearts. They are located one pair in each of 7th, 9th, 12th and 13th segments. The hearts of the 7th and 9th segments are called lateral hearts. The hearts of the 12, 13th segments are called latero-oesoph-ageal hearts. All the hearts connect the dorsal vessel with the ventral vessel. The latero-oesophageal hearts also open into the supra oesophageal vessel. The hearts are contractile. They pump the blood from the dorsal vessel into the ventral vessel.