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^thsegments 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:

Median longitudinal blood vessels
The intestinal blood plexus
The commissural vessel
The integumentary vessel
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:

Median longitudinal vessels
Herat and anterior loops
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 CO₂which are eliminate through nephridia, skin, and the coelomic fluid.

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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Circulatory system of earthworm