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
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 13th segment, Dorsal vessel is collective in function and the tributaries in every segment serves as the pathway for organs to receive blood.
- Commissural vessels
- Pairs of Dorso intestinal 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 13th) 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.
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 2nd 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 14th 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 14th 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 – 14th segment. They receive blood from stomach, body wall and septum through a pair of branches. They receive blood from seminal vesicle in the 11th and 12th segment.
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
There are a pair of blood vessels in the 10th and 11th 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.
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.
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 4th, 5th and 6th 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
Q1. Why does an earthworm need a closed circulatory system?
The closed circulatory system enables the faster transport of respiratory gases and nutrients. The blood cells present in the closed circulatory system have high oxygen binding capacity which helps in oxygenation even in poorly aerated environments.
Q2. How does an earthworm get oxygen?
Earthworms do not have lungs and respiratory tract to breathe in atmospheric oxygen. So they obtain oxygen from their environment like moist soil or water through their skin. Moist skin is essential for diffusion of oxygen which is provided by the mucus glands, coelomic fluid and nephridial excretions.
Q3. Which is the heart and Aorta of earthworm?
Anatomically, earthworms have 4 pairs of muscular thick walled dilatations with valves namely the lateral hearts and the lateral oesophageal hearts.
But functionally, the Dorsal vessel acts as the true heart in which the blood flow is from posterior to anterior and back flow is prevented by the valves. The ventral vessel act as the Aorta.
- Earthworm resources and vermiculture – Zoological survey of India
- Integrated principles of Zoology – Hickman, Cleveland P. Jr
- Biology of earthworms – Ayetan Karaca
- Rediscovering earthworms – C.S.K Mishra and Suryashika Samal