Circulatory System of the Earthworm

Earthworms are segmented worms and have a dual circulatory system – a closed circulation and an open circulation. The closed circulatory system is respiratory in function with hearts and blood vessels. Dorsal and Ventral vessels are the main longitudinal vessels.The open circulatory system has free moving coelomic fluid with suspended cells to provide immunity. 

Closed Circulatory System

The closed circulatory system consists of five main blood vessels which run lengthwise along the body, 4 pairs of hearts and smaller capillaries that form plexus in the tissues. Also, the branching, function and flow of blood vessels differ in the foregoing 13 segments than the remaining posterior (intestinal) segments. The main blood vessels are

• Dorsal vessel
• Ventral vessel
• Sub neural vessel
• A pair of Lateral neural/ oesophageal vessels

Dorsal Vessel

The Dorsal vessel is a single longitudinal tube that runs above the digestive tract from pharynx to anus. The presence of valves and its pumping action makes it the true heart of Earthworm. In the intestinal region i.e. posterior to 13^th segment, Dorsal vessel is collective in function and the tributaries in every segment serves as the pathway for organs to receive blood.

1. Commissural vessels
2. Pairs of Dorso intestinal vessels

Commissural vessels

Arise from Subneural vessels and the body wall where it gets blood, the prostate glands, and nephridia, and drain into the Dorsal vessel. They give off Septointestinal branches which proffer the intestine.

Two pairs of Dorsointestinal vessels

Collect blood from the intestine and drain into the Dorsal vessel. The blood flows from posterior to anterior in Dorsal blood vessels and the valves in every segment prevents the backflow of blood. 

The dorsal blood vessel becomes distributive in the antecedent segments (1 to 13^th) and the blood collected from the posterior segments are distributed directly over the pharyngeal bulb, the roof of the buccal chamber and the gizzard. Commissural and Dorsointestinal vessels are absent in antecedent segments. The Dorsal vessel is aborted anteriorly in some species.

Ventral Vessel

Above ventral nerve cord and below the alimentary canal is where you can find ventral vessel. The blood flow is antecedent to posterior starting from the 2^nd segment onwards. It emanates the following branches in every segment.

• A pair of ventro-tegumentaries in the antecedent 13 segments – supplies body wall, nephridia, septa and reproductive organs – gives Nephridial vessels to supply nephridia.
• Single median Ventro – intestinalis – supplies the lower intestines. This vessel is absent in antecedent segments

The Ventral vessel is thin walled and lacks the valves. It acts as the Aorta.

Sub Neural Vessel

The Sub neural vessel runs under the placement of ventral nerve cord from the 14^th segment onwards and carries blood starting at the anterior placement going to the posterior section. It receives blood from ventral body wall and nerve cord. It joins with the Dorsal vessel by a pair of commissural vessel and supply the intestine by the septo intestinal branch of commissural vessels. The Sub neural vessel is absent in several genera.

Lateral Oesophageal Vessel

At the 14^th segment, the Sub neural vessel divides to form a pair of lateral oesophageal vessels which lie loosely on the wall of alimentary canal in the antecedent 10 segments and lies closely connected to the stomach wall form 11 – 14^th segment. They receive blood from stomach, body wall and septum through a pair of branches. They receive blood from seminal vesicle in the 11^th and 12^th segment.

Supra-oesophageal vessel

It is the smallest transverse vessel lying on the superior aspect of stomach in the segments 9 – 13. It is a connecting vessel which receives blood from the oblique oesophageal vessels through the anterior loops and conduct the blood to ventral vessel via the lateral oesophageal hearts

Anterior loops

There are a pair of blood vessels in the 10^th and 11^th segment connecting the supraoesophageal vessel and the lateral oesophageal vessels. Theses loops are non – muscular, non – pulsatile and valve less. They receive blood from Lateral oesophageal vessel and send to Supraoesophageal vessel which then delivers the blood to Ventral vessel via the Lateral oesophageal hearts.

Hearts

There are 4 pairs of dilatations in the blood vessels, each in the antecedent segments of 7, 9, 12 and 13. They are thick walled muscular and pulsatile in nature. With each pulsation, the blood is propelled from above downwards i.e., starting at the Dorsal vessel going to the Ventral vessel. They are classified as

• Lateral hearts in segment 7 and 9
• Lateral oesophageal hearts in segment 12 and 13

The Lateral hearts has four valves and the Lateral oesophageal heart possess valves, specifically, three pairs that prevent the back flow of blood and ensures the downward flow. The lateral hearts send blood starting at the Dorsal vessel going to the Ventral vessel. The Lateral oesophageal heart send blood beginning at the Dorsal vessel and Supra oesophageal vessel to Ventral vessel

The position of the last pair of hearts is used to classify the different families, species and genera.

Capillary System

The capillaries are very small branches given off by the blood vessels at the tissue level for gaseous exchange. In Earthworms, the capillaries form extensive network on the external and internal surface of intestine to facilitate the absorption of nutrients. These are known as

• The External intestinal blood plexus
• The Internal intestinal blood plexus

The External intestinal blood plexus are situated on the outer surface of the intestine and are formed by the capillaries from the Ventro – intestinal branch of Ventral vessel. The blood from the External plexus is fed into the Internal plexus.

The Internal intestinal plexus is situated between the epithelial lining of the intestine and the circular muscle layer. It receives blood from the External intestinal plexus and is linked to the Dorsal vessel through the Dorso – intestinalis tributary. The main function of Internal plexus is to absorb the nutrients from the intestine.

Composition of Blood:

Earthworms have blood glands which synthesis the Blood cells and Haemoglobin. They are red colour follicles made up of syncytial wall enclosing a capsule that has loose cells. These glands are located in the 4^th, 5^th and 6^th segment right on top of the pharyngeal mass. They are developmentally related to pharyngeal nephridia and hence may be functions in excretory manner.

Haemoglobin is the respiratory pigment dissolved in plasma that carries oxygen and carbon dioxide to and fro in the tissues. It has good oxygen binding capacity that enables the respiration of Earthworm using its skin possible. The plasma also includes ameboid shaped cells haemocytes that play an important role in the plugging of broken vessels

Blood Circulation Pathway

The Dorsal vessel receives blood from intestine through the dorsointestinal vessel, Intestinal blood plexus and Commissural vessel and supply part of it to the pharynx, buccal cavity and gizzard and a major portion is conducted to Ventral vessel through the hearts.

The Subneural vessel collects blood from the ventral body wall, reproductive organs and prostate and conduct part of it to Dorsal vessel through commissural vessel and the rest to Ventral vessel through lateral and supraoesophageal vessels

The blood in the Ventral vessel is supplied to the intestine through ventrointestinal vessel and to the body wall, reproductive organs and nephridia through ventrotegumentaries.  

Blood flows beginning at the Dorsal to Ventral vessel through the hearts and lateral oesophageal to supraoesophageal vessel through the anterior loop.

Functions of Blood

The oxygen enters into the body by diffusion through the skin which is facilitated by the moisture provided by the mucus glands, coelomic fluid and the nephridial excretions. The oxygen from the skin is taken in to the circulation through the outer epidermal capillary plexus by binding it with haemoglobin in the plasma. Thus the circulating blood oxygenates the tissues

The blood from the alimentary canal contain nutrients and are distributed to various tissues. The blood from other parts contains waste products like CO₂ and nitrogenous waste which are then removed through nephridia, skin and coelomic fluid. 

The exchange of oxygen and CO₂ on the moist skin is facilitated by the delivering of these respiratory gases to the epidermal capillary plexus by the circulation blood. The circulatory system of Earthworm enables it to survive in both aerated and poorly aerated environment. They are also capable of drawing in oxygen from water.

Coelomic Circulatory System

The coelomic circulatory system consist of fluid and cells derived from the digestive system called coelomocytes. When the cells in the digestive system get replenished, the older cells which contain fat are released into the fluid filled coelomic cavity. 

The number of cells are more in the coelomic fluid than in the blood. In the coelom, these cells are free moving and are of different types providing for the nourishment, regeneration, immunity and wound healing. Thus the open circulatory system is mainly immune response related while the closed circulatory system is mainly respiratory in nature.

Video on circulatory system of earthworm : link

Frequently Asked Questions

References

1.  Earthworm resources and vermiculture – Zoological survey of India
2. Integrated principles of Zoology – Hickman, Cleveland P. Jr
3.  Biology of earthworms – Ayetan Karaca
4. Rediscovering earthworms – C.S.K Mishra and Suryashika Samal
5. https://en.wikipedia.org/wiki/Earthworm