Circulatory system of earthworm

Circulatory system of earthworm

• The circulatory or blood vascular system of an earthworm is a closed type.
• It consists of the blood vessel, heart, capillaries, and blood glands.
• Blood is composed of fluid plasma and colorless corpuscles.
• Blood is red in color due to the presence of respiratory pigment hemoglobin (erythrocruorin) in it.
• Hemoglobin is not contained in the corpuscles like vertebrates but occurs dissolved in plasma.
• Hemoglobin aids in the transportation of oxygen for respiration.

Blood vessels

They are of 2 types: collecting blood vessels and distributing blood vessels.

• They are closed tubes with a definite wall, and they break into capillaries to ramify in the different parts of the body.
• The arrangement of blood vessels in the anterior 13 segments is different from that behind the 13^th segments i.e., in the region of the intestine.
• So, the blood vessels can be described under 2 heads: A. Blood vessels and their arrangements behind the 13^th, i.e, intestinal region. B. Blood vessels and their arrangement in the anterior 13^th segments.

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A. Blood vessels and their arrangements behind 13^th, i.e. intestinal region

The blood vessels of this region include:

1. Median longitudinal blood vessels
2. The intestinal blood plexus
3. The commissural vessel
4. The integumentary vessel
5. The nephridial vessel

1. Median longitudinal blood vessels

a. Dorsal vessel

• The largest blood vessel of the body.
• Runs mid-dorsally above the alimentary canal, from one end of the body to another.
• Thickest vessel with contractile muscular walls visible from outside as a dark line through the thin and semitransparent body wall.
• Provided with pair of valves in the front of the septum in each segment.
• The direction of blood flow in this vessel is from backward to forward (from posterior to anterior).
• Contractile and pulsates rhythmically to force blood from posterior to anterior side.
• Behind the 13^th segment, it is collecting vessels, receiving blood through two pairs of dorso-intestinal vessels from the intestine, and a pair of commissural vessels from a sub-neural vessel in each segment.
• The commissural vessels from the loop behind each septum and they receive blood from the body wall, nephridia, and prostate glands.
• The commissural vessels also give out blood in each segment through a septointestinal branch to the intestine.

b. Ventral vessel

• It is also a large vessel that runs mid-ventrally below the alimentary canal and above the nerve cord from the 2^nd segment to the last segment of the body.
• Its walls are thin and without muscles and valves.
• The direction of blood flow in this vessel remains from anterior to the posterior side or from in front to backward.
• It is a principally distributing vessel. It supplies blood, each segment, through a pair of ventro-tegumentary vessels to integumentary nephridia, body wall, septa, and reproductive organs.
• This vessel also gives out a ventro-intestinal vessel to lower parts of the intestine in each segment behind the 13^th segment.
• Behind the 13^th segment, each ventro-tegumentary vessel sends a small branch, a septo-nephridial branch, supplying the septal nephridia.

c. Sub-neural vessel

• It is also a long and slender vessel that runs immediately beneath the nerve cord in the mid-ventral position.
• It extends from the 14^th segment to the last segment and is formed by the union of two lateral oesophageal vessels.
• It is without muscular walls and internal valves.
• The direction of blood flow is from in front backward.
• It is mainly a collecting vessel.
• It receives blood from the ventral nerve cord and ventral wall in each segment through a pair of small branches.
• It gives a pair of commissural vessels in each segment that joins the dorsal vessel.

2. Intestinal blood plexus

• The intestine is richly supplied with blood capillaries that form a close network.
• Consists of a close network of capillaries in the wall of the intestine.
• There are 2 capillaries networks in the intestine i) external plexus ii) internal plexus.
• External plexus lies on the surface of the gut and receives blood from the ventral vessel through ventro-intestinal and septo-intestinal and passes it on to internal plexus.
• The internal plexus is present in between the circular muscle layer of the intestine and the internal epithelial lining.
• Internal plexus passes on blood, along with absorbing nutrients, to the dorsal vessel through dorso-intestinal.

3. Commissural vessels

• It connects the dorsal and sub-neural vessels.
• They receive blood from nephridia, body wall, and the reproductive organs through capillaries and send them to the dorsal blood vessel.

4. Integumentary vessels

• These vessels coming from ventral vessels supply blood to integument for aeration.
• The aerated blood is collected by numerous capillaries of the commissural vessels in each segment.
• Thus, there is a close parallelism between venous and arterial capillaries throughout the body wall.

5. Nephridial vessels

• Originate from the ventro-tegumentary vessels of the ventral vessel.
• They supply blood to nephridia.

B. Blood vessels and their Arrangement in Anterior 13 segments

It consists of the following:

1. Median longitudinal vessels
2. Herat and anterior loops
3. Blood vessels of the gut

The function of collecting blood from the anterior region of the gut is taken over by a new vessel supra-oesophageal, while the blood from the peripheral structures is collected by the right and left lateral oesophageal.

1. Median longitudinal blood vessels

a. Dorsal vessel

• The blood vessel becomes the distributing vessel in these segments instead of the collecting vessel.
• Structurally it is like that of anterior segments.
• But it has neither dorso-intestinal nor commissural vessels opening into it.
• It sends out all the collected blood from the posterior region of the body into the hearts and the anterior region of the gut.
• In the gut it divides into 3 branches distributes over the pharyngeal bulb and the roof of the buccal chamber.
• However, it supplies to the stomach, gizzard, esophagus, pharynx, and other related parts.

b. Ventral vessel

• The blood vessel remains to distribute in these segments also but extends only up to the second segment.
• No ventro-intestinal, hence it does not supply to the alimentary canal in this region.
• Ventral vessels give off a pair of ventro-tegumentary vessels in each segment supply blood to the body wall, septa, nephridia, and reproductive organs.

c. Supra- oesophageal vessel

• It is the shortest, thin-walled collecting vessel lying mid-dorsally above the stomach and confined to segments 9 to 13.
• Connected to the lateral oesophageal vessel through 2 pairs of anterior loops.
• Connected to the ventral vessel through 2 pairs of latero-oesophageal hearts.
• At places, it divides into separate vessels that reunite to form a single vessel.
• It collects blood from the stomach, gizzard, and (through anterior loops) from lateral oesophageal.
• And pumps it through lateral oesophageal hearts into the ventral vessel.

d. Lateral oesophageal

• In fact, the subneural vessels bifurcate in the 14^th segments to form 2 lateral oesophageal.
• These vessels are thick and lie one on the ventrolateral side of the gut, running from the anterior end of the body up to the 13^th segment.
• Closely attached to the wall of the stomach from 10^th to 13^th segments and communicate with the ring vessels.
• These receive a pair of ventro-tegumentary vessels in each segment, collecting blood from the body wall, septa, nephridia, and reproductive organs.
• Some of the blood passes to supra-oesophageal vessels through anterior loops in each of the segments 10 and 11 and through several ring vessels running through the wall of the stomach.
• The rest of the blood flows back into the sub-neural vessels.
• They function likes subneural and commissural vessels of the posterior region e. these are collecting vessels.

2. Hearts and anterior loop

• In each segment 7, 9, 12, and 13 is found a pair of large, thick, muscular, and rhythmically contractile vertical vessels, called hearts.
• They are neurogenic i.e., the heart originates in the nerve cells of the heart.
• They pump blood from dorsal to the ventral vessels, while flow in opposite direction is prevented by internal valves.
• The hearts of the 12^th and 13^th segments are joined above to both the dorsal and the oesophageal vessels, called latero-oesophageal hearts.
• These hearts have thick muscular walls and a pair of valves at each junction with dorsal vessels and supra-oesophageal vessel, and another pair of valves at the ventral end.
• These allow flowing blood downwards only.
• Another heart of 7^th and 9^th segments connect dorsal and ventral vessels only and are called lateral hearts.
• They have 4 pairs of valves that allow blood to flow only downwards.
• Besides 4 pairs of heart, there are 2 pairs of loop-like vessels called anterior loop.
• The anterior loop is a pair of thin-walled, non-pulsatile, non-muscular, and loop-like broad vessels, without valves, in each of the 10th and 11th segments.
• Anterior loop covey blood from lateral-oesophageal into the supra-oesophageal vessel.

3. Blood vessels of the gut

• On another side of the stomach situated ring-like vessels.
• Ring vessels are characteristic circular vessels of the stomach situated within the muscular coat, about 12 vessels per segment.
• These vessels connect the supra-oesophageal and lateral-oesophageal vessels.
• Through these vessels, blood flows upwards from the lateral-oesophageal into the supra-oesophageal.
• The buccal cavity, pharynx, and gizzard receive their blood supply from dorsal blood vessels directly.

Video: Circulatory system in Earthworm by Studio Biology.

Circulation of blood

• Blood flows from behind to forward in the dorsal vessel.
• And from front to backward in ventral, latero-oesophageal, supra-oesophageal, and sub-neural vessels.
• The ventral vessel is the main distributing vessel, supplying blood to all parts of the body.
• In the first 13 segments, it supplies blood to the body wall, septa, nephridia, and reproductive organs through ventro-tegumentary.
• Behind the 13^th segment, it supplies blood to the body wall and nephridia through ventro-tegumentary.
• Supplies blood to gut wall through ventro-intestinal.
• Sub-neural, lateral oesophageal, and supra-oesophageal are the main collecting vessels.
• Lateral oesophageal collects blood in the first 13 segments from the alimentary canal, body wall, nephridia, septa, and reproductive organs.
• And they discharge into supra-oesophageal through anterior loops and ring vessels.
• Supra-oesophageal also collects blood from the gizzard and stomach and pours into the ventral vessel through latero-oesophageal hearts.
• Sub-neural collects blood in the intestinal region from the ventral body wall and nerve cord.
• And send into dorsal vessels through the commissural which also receives blood from the body wall, septa and nephridia.
• Commissural also pours some blood into the gut wall through septo-intestinal.
• Dorsal vessel functions both collecting and distributing vessels.
• It collects blood through dorso-intestinal in the intestinal region from the gut wall.
• And through commissural from sub- neural vessel, septa, and nephridia.
• In the first 13 segments, it distributes some blood through branches to the alimentary canal and pours the remaining blood through hearts into the ventral vessel.
• The blood distributes digested food to various body regions.
• And it collects waste substances like nitrogenous waste and CO₂ which are eliminate through nephridia, skin, and the coelomic fluid.

Read Also: Nervous System of Earthworm

Blood glands

• In the segments, 4, 5, and 6 segments above pharyngeal mass and connected with pharyngeal or salivary glands are found small, red-colored, follicular bodies, the blood glands.
• Each gland consists of a mass of loose cells surrounded by a capsule with a syncytial wall.
• These glands are connected with pharyngeal nephridia and with salivary glands.
• These glands manufacture blood corpuscles and hemoglobin.
• They are also regarded to be excretory by some workers.

References and Sources

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