Circulatory System of Earthworm

The earthworm possesses a closed type of circulatory system, which plays a pivotal role in the transportation of nutrients, oxygen, and waste products throughout its body. This system is intricate and is composed of various components, each with its specific function.

Blood Composition:
- The blood of an earthworm is red due to the presence of the respiratory pigment called hemoglobin, also known as erythrocruorin. Unlike vertebrates, in earthworms, hemoglobin is dissolved directly in the plasma rather than being contained within blood cells. This hemoglobin is crucial for transporting oxygen required for respiration. Besides hemoglobin, the plasma contains colorless corpuscles, each with a nucleus.
Blood Vessels:
- The blood vessels in earthworms can be categorized into two main types: collecting blood vessels and distributing blood vessels. These vessels are closed tubes with defined walls. They further branch out into smaller vessels called capillaries, which extend to various parts of the earthworm’s body.

Arrangement of Blood Vessels:
- The organization of these blood vessels varies depending on their location within the earthworm. Specifically, the vessels in the anterior thirteen segments have a different arrangement compared to those located behind the thirteenth segment, in the intestinal region. Therefore, for a comprehensive understanding, the blood vessels can be studied under two primary sections:
  - Blood vessels and their arrangement in the segments behind the 13th, specifically in the intestinal region.
  - Blood vessels and their configuration in the anterior thirteen segments.
Function of Hemoglobin:
- Hemoglobin serves a vital function in the earthworm’s circulatory system. It aids in the transportation of oxygen, ensuring that all parts of the earthworm receive the necessary oxygen for metabolic processes.

A. Blood Vessels behind 13th segments in Intestinal Region

In the anatomy of the earthworm, particularly in the segments located behind the 13th segment, the circulatory system exhibits a unique arrangement tailored to support the functions of the intestinal region. This area is vital for the digestion and absorption of nutrients. Therefore, the blood vessels in this region play a crucial role in transporting nutrients and oxygen to support these processes.

Median Longitudinal Blood Vessels:
- These are primary vessels that run lengthwise through the earthworm’s body. They are centrally located and serve as the main channels for blood flow in the intestinal region. Their strategic placement ensures efficient distribution and collection of blood throughout the segment.

Intestinal Blood Plexus:
- This is a complex network of smaller blood vessels that surrounds the intestine. The plexus is essential for the direct exchange of nutrients and waste products between the blood and the digestive system. It ensures that digested nutrients are promptly absorbed into the bloodstream for transportation to other parts of the body.
Commissural Vessel:
- These vessels act as connectors between the major blood vessels. They play a role in ensuring that blood circulates effectively throughout the intestinal region, facilitating the even distribution of oxygen and nutrients.
Integumentary Vessel:
- Located closer to the outer layer or skin of the earthworm, these vessels are responsible for transporting blood to the integument or the outer covering. They play a pivotal role in oxygen exchange, given that earthworms respire through their skin.

Nephridial Vessel:
- These vessels are associated with the nephridia, the excretory organs of the earthworm. They ensure that waste products are efficiently removed from the blood and excreted out of the body. Their function is vital for maintaining the internal balance and health of the earthworm.

1. Median Longitudinal Blood Vessels

Among the various blood vessels, the median longitudinal blood vessels are of paramount importance. These vessels are strategically positioned and are essential for the earthworm’s survival and functioning.

Dorsal Blood Vessel:
- Situated above the intestine in the mid-dorsal line, the dorsal blood vessel is the largest and thickest in the earthworm. Its muscular, contractile walls enable it to rhythmically pump blood from the posterior to the anterior side.
- This vessel is visible as a dark line due to the thin and semi-transparent body wall of the earthworm.
- Internally, in each segment, it possesses a pair of valves that prevent the backward flow of blood, ensuring unidirectional circulation.
- Often regarded as the true heart in earthworms, the dorsal blood vessel functions as a distributive vessel in segments 1-13 and as the primary collecting vessel from segment 14 onwards.
- It gathers blood from various sources, including dorso-intestinal vessels, commissural vessels, and more.

Ventral Blood Vessel:
- Located below the alimentary canal and above the ventral nerve cord, the ventral blood vessel is the main distributive vessel in the earthworm.
- Unlike the dorsal vessel, it has thin walls and lacks muscles and valves.
- Blood flows from the anterior to the posterior side in this vessel.
- In the anterior segments, it supplies blood to various organs, including the body wall, nephridia, and reproductive organs. In the intestinal region, it provides blood to the intestine through ventro-intestinal vessels.
Sub-neural Vessel:
- Positioned mid-ventrally below the ventral nerve cord, the sub-neural vessel extends from the 14th segment to the last segment.
- This vessel primarily functions as a collecting vessel, gathering blood from the ventral body wall and supplying some to the intestine.
- It lacks muscular walls and internal valves, ensuring a smooth flow of blood from the anterior to the posterior end.

2. Intestinal Blood Plexus

A significant component of this system, especially in the digestive region, is the intestinal blood plexus. This plexus plays a pivotal role in the absorption and transportation of nutrients from the intestine.

Overview of the Intestinal Blood Plexus:
- The intestinal blood plexus is a dense network of capillaries located within the wall of the intestine. These capillaries are essential for supplying the intestine and facilitating the absorption of digested nutrients.

External and Internal Plexus:
- The intestinal blood plexus can be categorized into two main networks: the external plexus and the internal plexus.
- The external plexus is situated on the outer surface of the intestine. It receives blood from the ventral vessel through the ventro-intestinal branches. Afterward, the blood is channeled to the internal plexus.
- The internal plexus, on the other hand, is located between the circular muscle layer of the intestine and its internal epithelial lining. This network is directly connected to the dorsal blood vessel via the dorso-intestinals.
Function of the Internal Plexus:
- The primary function of the internal plexus is to absorb nutrients from the gut. As the earthworm digests its food, the nutrients are broken down into simpler forms. The internal plexus, with its close network of capillaries, efficiently absorbs these nutrients, ensuring they are transported to various parts of the earthworm’s body for sustenance and growth.

3. Commissural Vessels

In the intricate circulatory system of earthworms, the commissural vessels play a pivotal role in ensuring the efficient flow and distribution of blood. These vessels serve as crucial connectors and transporters within the system.

Connection and Linkage:
- The primary function of the commissural vessels is to link the dorsal and sub-neural vessels. This connection ensures a streamlined flow of blood between these two major vessels, optimizing the distribution of blood throughout the earthworm’s body.

Blood Reception:
- Besides their role as connectors, commissural vessels have a vital function in receiving blood. They obtain blood from various parts of the earthworm’s body, including the nephridia, body wall, and reproductive organs. This blood is collected through a network of capillaries, ensuring efficient and effective blood collection.
Supply to the Dorsal Blood Vessel:
- After receiving blood from various sources, the commissural vessels then have the responsibility of supplying this blood to the dorsal blood vessel. This ensures that the collected blood is efficiently circulated throughout the earthworm’s body, reaching all necessary organs and tissues.
Significance of Function:
- The dual role of the commissural vessels, both as connectors and transporters, emphasizes their importance in the earthworm’s circulatory system. By ensuring the efficient flow of blood between major vessels and collecting blood from various sources, they play a crucial role in maintaining the health and functionality of the earthworm.

4. Integumentary Vessels

Among the various components of this system, the integumentary vessels play a pivotal role in the aeration of blood.

Origin and Function:
- Integumentary vessels have their origin from the ventral vessels. Their primary function is to supply blood to the integument, which is the outermost layer of the earthworm’s body. This process is crucial for aeration, ensuring that the blood receives the necessary oxygen.

Aeration Process:
- As the integumentary vessels supply blood to the integument, the blood undergoes aeration. This means that the blood absorbs oxygen from the surrounding environment, vital for the earthworm’s metabolic processes and overall survival.
Collection of Aerated Blood:
- Once the blood is aerated, it doesn’t remain in the integument. Instead, it is collected by a myriad of capillaries associated with the commissural vessels present in each segment of the earthworm’s body.
Parallelism in the System:
- An interesting observation is the close parallelism between the venous and arterial capillaries throughout the earthworm’s body wall. This parallel structure ensures efficient and streamlined blood flow, optimizing the aeration process and subsequent transportation of aerated blood.

5. Nephridial Vessels

In the complex circulatory system of earthworms, the nephridial vessels hold a specific and essential role, particularly concerning the nephridia, the excretory organs of the earthworm.

Origin and Source:
- The nephridial vessels have their origins in the ventro-tegumentary vessels, which are branches of the primary ventral vessel. This connection ensures that the nephridial vessels receive a consistent and adequate supply of blood from the main circulatory system.

Function and Purpose:
- The primary function of the nephridial vessels is to supply blood to the nephridia. Nephridia are vital excretory organs in earthworms, responsible for the removal of waste products from the body. Therefore, the supply of blood to these organs is crucial for the efficient functioning of the excretory system.
Significance in Circulation:
- By ensuring that the nephridia receive a steady flow of blood, the nephridial vessels play a pivotal role in maintaining the health and balance of the earthworm’s internal environment. The blood supplied to the nephridia carries essential nutrients and oxygen, while also facilitating the removal of waste products.
Emphasis on Function:
- The specific role of the nephridial vessels in supplying blood to the nephridia underscores their importance in the overall circulatory and excretory systems of the earthworm. Their function ensures that the nephridia can operate efficiently, maintaining the internal balance and health of the earthworm.

B. Blood Vessels anterior to 13 Segments

In the anterior 13 segments of an earthworm, the circulatory system is specialized and distinct from the rest of the body, reflecting the different physiological demands of this region. The system here is composed of median longitudinal vessels, hearts and anterior loops, and blood vessels of the gut.

Median Longitudinal Vessels:
- These vessels run longitudinally and are central to the distribution and collection of blood in the anterior segments. They are crucial for transporting nutrients and oxygen to the tissues in these segments.

Hearts and Anterior Loops:
- The earthworm has pseudohearts, also known as aortic arches, located in segments 7 to 11. These structures pump blood and maintain pressure within the circulatory system. The anterior loops are associated with these pseudohearts and help in circulating blood to the upper part of the earthworm’s body.
Blood Vessels of the Gut:
- These vessels are involved in supplying blood to the gut region in the anterior segments. They are essential for the digestive process, providing the gut tissues with necessary oxygen and nutrients, and carrying away metabolic waste.
Supra-Oesophageal Vessel:
- This vessel takes over the function of collecting blood from the anterior region of the gut. It is a significant component of the circulatory system in these segments, ensuring that blood is efficiently returned to the dorsal blood vessel.

Lateral Oesophageal Vessels:
- The right and left lateral oesophageal vessels are responsible for collecting blood from the peripheral structures. These vessels ensure that blood from the body wall and other peripheral tissues is collected and directed back into the main circulatory system.

The arrangement of these vessels in the anterior segments is critical for the earthworm’s survival, as it ensures that the head region, which includes the brain and sensory organs, receives an adequate supply of blood. This specialized network of vessels supports the high metabolic demand of the anterior segments, which are involved in activities such as feeding, locomotion, and sensory processing.

1. Median Longitudinal Blood Vessels

In the anterior 13 segments of an earthworm, the arrangement of the median longitudinal blood vessels plays a pivotal role in the circulatory system. These vessels are central to the distribution and collection of blood in these segments, ensuring efficient transportation of nutrients and oxygen to the tissues.

Dorsal Blood Vessel:
- Located in the anterior 13 segments, the dorsal blood vessel primarily functions as a distributive vessel.
- Structurally, it is similar to its counterpart in the posterior segments but lacks dorso-intestinalis and commissural vessels opening into it.
- This vessel is responsible for exporting all the collected blood from the posterior region of the body into the hearts and the anterior region of the gut.
- It branches off into three distinct branches that are distributed over the pharyngeal bulb and the roof of the buccal chamber.
- Furthermore, it supplies blood to various parts of the digestive system, including the stomach, gizzard, oesophagus, and pharynx.
Ventral Blood Vessel:
- In the anterior segments, the ventral blood vessel also serves a distributive function.
- It extends up to the second segment.
- Unlike in the posterior segments, due to the absence of ventrointestinals, this vessel does not supply blood to the alimentary canal in the anterior region.
- However, in the first 13 segments, it supplies blood to various structures, including the body wall, septa, nephridia, and reproductive organs, through the ventro-tegumentaries.

Supra-Oesophageal Vessel:
- This vessel, which is the smallest in the body, spans from the 9th to the 13th segment.
- Positioned above the stomach, it is the main transverse vessel in the first 13 segments.
- It receives blood from the lateral oesophageals through two pairs of anterior loops that surround the stomach in the 10th and 11th segments.
- Its primary function is to transport the collected blood by the latero-oesophageal hearts in the 12th and 13th segments to the ventral vessel.
Lateral Oesophageals:
- Originating in the 14th segment, the sub-neural vessel bifurcates to form the two lateral oesophageals.
- In the anterior thirteen segments, these vessels are thick and are closely associated with the ventro-lateral margins of the alimentary canal.
- They are tightly connected to the stomach wall from the 10th to the 13th segments and interact with the ring vessels.
- Moving forward, they exist freely from the wall of the alimentary canal and continue to receive branches in each segment.
- These vessels play a crucial role in collecting blood from various structures, including the body wall, septum, reproductive organs, and nephridia.

2. Hearts and Anterior Loops

In the anterior region of an earthworm, specifically in the segments leading up to the 13th segment, the dorsal and ventral blood vessels are intricately connected by four pairs of pulsatile hearts. These hearts play a pivotal role in the circulatory system of the earthworm, ensuring efficient transportation of blood between the dorsal and ventral vessels.

Hearts:
- Unlike the posterior segments, where the dorsal and ventral blood vessels have no direct connections, the anterior region boasts four pairs of hearts that bridge these vessels.
- These hearts are neurogenic, meaning the heartbeat originates in the nerve cells of the heart.
- These contractile structures encircle the alimentary canal and are located in the 7th, 9th, 12th, and 13th segments.
- Each heart is equipped with valves that ensure a unidirectional flow of blood.
Lateral Hearts:
- Located in the 7th and 9th segments, the lateral hearts possess four pairs of valves that permit blood to flow only downwards.
- Their primary function is to transport blood from the dorsal vessel to the ventral vessel.
Lateral Oesophageal Hearts:
- These are found in the 12th and 13th segments.
- Characterized by their thick muscular walls, each lateral oesophageal heart is equipped with three pairs of valves.
- They facilitate the flow of blood from the supraoesophageal and dorsal vessel to the ventral vessel.
- The valves at each junction with the dorsal vessels and supra-oesophageal vessel, along with the pair at the ventral end, ensure the blood flows only in a downward direction.
Anterior Loops:
- In addition to the hearts, the anterior region also contains two pairs of loop-like vessels known as anterior loops.
- These non-muscular, non-pulsatile structures lack valves and are found in the 10th and 11th segments.
- Their primary function is to convey blood from the lateral oesophageal vessel to the supraoesophageal vessel, eventually directing it into the ventral vessel through the hearts of the 12th and 13th segments.

3. Blood Vessels of the Gut

The blood vessels of the gut play a pivotal role in ensuring the efficient circulation of blood within the alimentary canal of an earthworm. These vessels are intricately designed to cater to the specific needs of the gut and its associated organs.

Ring Vessels:
- Situated on the other side of the stomach, the ring vessels are characteristic circular vessels embedded within the muscular coat of the stomach.
- Typically, about 12 vessels are present per segment, emphasizing the dense network of these vessels in this region.
- Their primary function is to connect the supra-oesophageal and lateral-oesophageal vessels, establishing a bridge between these two major vessels.
- Through the ring vessels, blood flows in an upward direction, moving from the lateral-oesophageal vessels into the supra-oesophageal vessels. This ensures a continuous and efficient circulation of blood within the gut region.
Blood Supply to Buccal Cavity, Pharynx, and Gizzard:
- Besides the ring vessels, the buccal cavity, pharynx, and gizzard have their own dedicated blood supply.
- These organs receive blood directly from the dorsal blood vessels. This direct connection ensures that these organs receive a consistent and adequate supply of oxygenated blood, which is crucial for their proper functioning.

How is blood circulated in earthworm?

The circulation of blood in an earthworm is a complex and efficient system that ensures the distribution of nutrients and the removal of waste products. The process involves various vessels and structures that work in tandem to maintain the worm’s health and vitality.

Dorsal Vessel:
- The dorsal vessel collects blood through dorso-intestinals from the gut wall in the intestinal region.
- It also receives blood from the sub-neural vessel, septa, and nephridia through commissurals.
- In the first 13 segments, the dorsal vessel distributes some blood to the alimentary canal and directs the remaining blood through the hearts to the ventral vessel.
- Blood flows from the posterior to the anterior region in the dorsal vessel.
Ventral Vessel:
- The ventral vessel is the primary distributing vessel, supplying blood to all parts of the earthworm’s body.
- In the initial 13 segments, it provides blood to the body wall, septa, nephridia, and reproductive organs via ventro-tegumentary.
- Beyond the 13th segment, it supplies blood to the body wall and nephridia through ventro-tegumentary and to the gut wall via ventro-intestinal.
- Blood flows from the anterior to the posterior region in the ventral vessel.
Sub-neural and Lateral Oesophageal Vessels:
- The sub-neural vessel collects blood from the ventral body wall and nerve cord in the intestinal region.
- It directs this blood to the dorsal vessel through the commissurals.
- The lateral oesophageal vessels collect blood from various parts, including the alimentary canal, body wall, nephridia, septa, and reproductive organs in the first 13 segments.
- This collected blood is then transferred to the supra-oesophageal vessel through anterior loops and ring vessels.
Supra-oesophageal Vessel:
- This vessel collects blood from the gizzard and stomach.
- It then directs this blood to the ventral vessel through the latero-oesophageal hearts.
Function of Blood in Earthworm:
- The primary function of blood in an earthworm is to distribute digested food to various body regions.
- Additionally, it collects waste substances, including nitrogenous waste and CO2.
- These waste products are then eliminated through the nephridia, skin, and coelomic fluid.

Functions of blood in earthworm

The blood in an earthworm plays a crucial role in maintaining its physiological functions and overall health. The following are the primary functions of blood in an earthworm:

Distribution of Nutrients:
- The blood is responsible for distributing digested food to various regions of the earthworm’s body. This ensures that all parts of the earthworm receive the necessary nutrients for their proper functioning and growth.
Waste Removal:
- Besides distributing nutrients, the blood also absorbs waste materials. These waste materials include nitrogenous waste and carbon dioxide (CO2).
- Once absorbed, these waste products are directed to specific organs for elimination. The nephridia, skin, and coelomic fluid play a pivotal role in this waste removal process.
Respiration:
- Respiration in earthworms primarily occurs through the integument or skin. This process involves the diffusion of gases through the integument.
- The integument contains a capillary network in its outer epidermal layer, especially in larger forms of earthworms. This network facilitates the exchange of gases.
- For effective gas diffusion, a film of moisture is essential. This moisture is provided by various sources, including mucous glands, coelomic fluid, and nephridial excretions.
Oxygen Absorption:
- Plasma haemoglobin present in the blood absorbs oxygen (O2) from the capillaries of the skin. However, for this absorption to occur, the skin must be moist so that O2 can mix with haemoglobin.
- Haemoglobin is a specialized pigment that can absorb O2 from both the surrounding air and oxygen-deficient environments. This adaptability ensures that the earthworm receives a consistent supply of oxygen, regardless of its environment.
Survival in Varied Environments:
- Earthworms have the ability to survive in well-aerated water without drowning. This is due to their efficient respiratory system and the presence of haemoglobin in their blood.
- Additionally, earthworms can endure oxygen-deprived conditions for several hours. During this time, they may resort to anaerobic respiration to sustain their energy needs.

Blood Glands in Earthworm

In the anatomy of the earthworm, blood glands play a significant role in the synthesis and maintenance of the blood system. The following provides a detailed explanation of the blood glands in an earthworm:

Location and Structure:
- Blood glands are situated in specific segments of the earthworm, specifically in the 4th, 5th, and 6th segments. These segments are located above the pharyngeal mass.
- Structurally, blood glands comprise various groups of small, rounded follicles that exhibit a distinct red coloration. This red hue is indicative of their association with blood and hemoglobin synthesis.
- Each follicle is characterized by a syncytial wall that encloses a capsule. Within this capsule is a mass of loosely arranged cells.
Primary Function:
- The primary function of blood glands is the synthesis of vital components of the blood system. They are responsible for producing blood corpuscles and hemoglobin.
- Hemoglobin is a crucial protein that binds to oxygen and facilitates its transport throughout the body of the earthworm. Blood corpuscles, on the other hand, play a role in carrying nutrients and removing waste.
Association with Other Organs:
- Blood glands have a close association with pharyngeal nephridia and salivary glands. This relationship suggests that blood glands might have functions beyond just blood synthesis.
- Given their connection with pharyngeal nephridia, it is postulated that blood glands may also have an excretory function. This means that they might play a role in the removal of waste products from the earthworm’s body.

Lymph glands in Earthworm

In the intricate anatomy of the earthworm, lymph glands play a pivotal role in the immune system. Here’s a detailed exposition on the lymph glands in an earthworm:

Location and Structure:

Lymph glands are strategically situated on both sides of the dorsal blood vessel. They are specifically found from the 26th segment and extend to the segments behind it.
In terms of structure, each segment from the 26th segment onwards possesses one pair of lymph glands. These glands are small in size and exhibit a whitish hue, making them distinct from other structures in the vicinity.

Primary Function:

The primary function of the lymph glands is the production of specific cells that play a crucial role in the earthworm’s immune response.
These glands are responsible for producing phagocytic cells. Phagocytic cells are specialized cells that engulf and digest harmful foreign particles, bacteria, and dead or dying cells. This process is known as phagocytosis.
Therefore, the presence of these phagocytic cells indicates that the lymph glands are vital for the earthworm’s defense mechanism against potential pathogens or harmful entities.

Significance:

The presence of lymph glands underscores the earthworm’s capability to ward off infections and maintain its overall health.
By producing phagocytic cells, the lymph glands ensure that any foreign invaders are promptly identified and eliminated, safeguarding the earthworm from potential diseases.

References

https://www.onlinebiologynotes.com/circulatory-system-of-earthworm/
https://www.notesonzoology.com/earthworm/circulatory-system-in-earthworm/2073
https://eduinput.com/circulatory-system-of-earthworm/
https://laboratoryinfo.com/circulatory-system-of-the-earthworm-diagram/
https://rajusbiology.com/earthworm-circulatory-system/