Earthworm Circulatory System: Structure, Blood Vessels, Hearts & Blood Flow Mechanism | BSc Zoology Notes

Circulatory System of Earthworm 

• The earthworm circulatory system is a well-developed closed circulatory system, in which blood always remains confined within blood vessels. 
• This system is responsible for the transportation of respiratory gases, nutrients, metabolic wastes, and hormones throughout the body. 
• The circulatory system consists of three main components: blood, blood vessels, and hearts.

1. Blood

• The blood of earthworm is reddish in colour due to the presence of the respiratory pigment haemoglobin. 
• Unlike vertebrates, haemoglobin is not enclosed within red blood cells but remains dissolved directly in the plasma. 
• The blood consists of a fluid plasma and colourless, nucleated, amoeboid corpuscles. These corpuscles perform defensive and phagocytic functions. 
• The dissolved haemoglobin facilitates the transport of oxygen and carbon dioxide throughout the body.

2. Blood Vessels

The circulatory system contains a network of blood vessels that transport blood to different organs and tissues.

Dorsal Blood Vessel

• The dorsal blood vessel is the largest blood vessel of the earthworm and lies above the alimentary canal. It is thick-walled, contractile, and provided with valves that prevent the backward flow of blood. Blood flows anteriorly through this vessel.
• In the region anterior to the 13th segment, the dorsal vessel acts mainly as a distributing vessel, supplying blood to the ventral vessel through the hearts and also sending branches to the pharynx, oesophagus, gizzard, buccal cavity, and pharyngeal nephridia.
• Posterior to the 13th segment, it functions primarily as a collecting vessel, receiving blood from the intestine and other body regions before conveying it forward.

Ventral Blood Vessel

• The ventral blood vessel runs beneath the alimentary canal throughout the length of the body. It is thin-walled, non-contractile, and lacks valves. Blood flows posteriorly through this vessel.
• It acts as the principal distributing vessel of the body and supplies blood to the body wall, nephridia, intestine, reproductive organs, and other tissues through its branches.

Subneural Blood Vessel

• The subneural blood vessel is situated beneath the ventral nerve cord and extends from the 14th segment to the posterior end of the body. It is formed by the union of the lateral oesophageal vessels.
• Blood flows backward through this vessel. It acts mainly as a collecting vessel, receiving blood from the nerve cord and ventral body wall. The collected blood is ultimately transferred to the dorsal vessel through commissural vessels.

Supra-oesophageal Blood Vessel

• The supra-oesophageal vessel is a median vessel situated above the oesophagus and stomach between the 9th and 13th segments. It collects blood from the stomach and gizzard region. This vessel receives blood from the lateral oesophageal vessels through anterior loops and supplies blood to the posterior pairs of hearts.

Lateral Oesophageal Vessels

• These are paired vessels located on either side of the alimentary canal and extend from the anterior region to the 13th segment. 
• They collect blood from the body wall, septa, nephridia, and reproductive organs. Posteriorly, they unite to form the subneural vessel. 
• They are connected to the supra-oesophageal vessel by anterior loops.

Other Blood Vessels

Several smaller vessels assist in circulation:

• Anterior loops connect the lateral oesophageal vessels with the supra-oesophageal vessel.
• Ring vessels form circular connections around the gut.
• Ventro-tegumentary vessels arise from the ventral vessel and supply the body wall and associated organs.
• Commissural vessels connect the subneural vessel with the dorsal vessel.
• Dorso-intestinal vessels collect blood from the intestine and transfer it to the dorsal vessel.
• Ventro-intestinal vessels carry blood from the ventral vessel to the intestine.
• Intestinal plexus consists of a network of blood vessels in the intestinal wall that ensures efficient exchange of nutrients and gases.

3. Hearts

• Earthworms possess four pairs of hearts, making a total of eight hearts. These are situated in the 7th, 9th, 12th, and 13th segments.
• The hearts present in the 7th and 9th segments are known as lateral hearts, while those present in the 12th and 13th segments are called latero-oesophageal hearts.
• All hearts are muscular and contractile. Their primary function is to pump blood from the dorsal vessel into the ventral vessel.
•  The latero-oesophageal hearts are additionally connected with the supra-oesophageal vessel. Valves are present within the hearts to prevent the reverse flow of blood and ensure unidirectional circulation.

Mechanism of Blood Circulation

• Blood collected from the posterior region of the body enters the dorsal vessel through various collecting vessels. The dorsal vessel carries blood forward toward the anterior end of the body. The hearts pump this blood into the ventral vessel. From the ventral vessel, blood is distributed to all organs and tissues through its branches.
• After supplying oxygen and nutrients to the tissues, blood is recollected through various vessels such as the lateral oesophageal vessels, subneural vessel, and dorso-intestinal vessels. It finally returns to the dorsal vessel, thus completing the circulation cycle.

The direction of blood flow is:

Dorsal vessel	Forward
Ventral vessel	Backward
Subneural vessel	Backward
Lateral oesophageal vessels	Backward
Supra-oesophageal vessel	Backward

• Valves present in the dorsal vessel and hearts prevent the backward flow of blood and maintain efficient circulation.

Conclusion

The circulatory system of Pheretima is a highly organized closed vascular system consisting of blood, blood vessels, and four pairs of hearts. The dorsal vessel acts mainly as a collecting vessel posteriorly and a distributing vessel anteriorly, while the ventral vessel serves as the chief distributing vessel. The presence of dissolved haemoglobin in plasma and multiple pairs of hearts ensures efficient transport of oxygen, nutrients, and wastes, enabling the earthworm to maintain its physiological activities effectively.

Referrence

• Kotpal RL - Modern Textbook of Zoology Invertebrates
• Jordan and Verma - Invertebrate Zoology
• Barnes - Invertebrate Zoology

Suggested Reading

• Digestive System of Earthworm