Circulatory System of Earthworm
Circulatory System of Earthworm
The circulatory system or blood vascular system of earthworms ( Pheretima posthuma and most of the terrestrial oligochaetes are similar) is closed type ( blood is present within blood vessels and circulates unidirectionally from the heart to the body and back again).
It comprises
- blood,
- blood vessels and
- capillaries,
which transport blood to all parts of the body.
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| Circulatory System of Earthworm Diagram |
Blood:
The blood is composed of
- Fluid plasma: The red respiratory pigment haemoglobin or erythrocruorin is dissolved in plasma which gives a red colour and functions in the transportation of oxygen for respiration.
- Corpuscles are colourless and physiologically similar to leucocytes of vertebrates.
Blood Vessels:
There are three types.
- Longitudinal blood vessels.
- Lateral or transverse blood vessels.
- Intestinal plexus.
Longitudinal blood vessels:
- Dorsal vessel.
- Ventral vessel.
- Lateral oesophageal vessels.
- Sub-neutral vessel.
- Supra-oesophageal blood vessel.
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| Circulatory System of Earthworm ( Blood Vessels ) Schematic Diagram |
I. Dorsal vessel :
Hearts: In each of the segments, 7, 9, 12 and 13 is found a pair of wide pulsating, rhythmically contractile, vertical vessels containing valves to prevent backflow.
Hearts of the 7th and 9th segments are called lateral hearts and connect to the dorsal and ventral Vessels only.
Those of the 12th and 13th segments connect both dorsal and supra Oesophageal with ventral vessels and are called latero-oesophageal hearts.
The function of ' hearts':
The function of the 'hearts' is to pump the blood down into the large ventral vessels in which the blood flows forward anteriorly to the head and backwards posteriorly distributing it to all parts of the body.
3. The dorsal vessel trifurcates to distribute three branches over the pharyngeal bulb and roof of the buccal chamber.
4. In each of the 3rd, 4th, 5th, 6th and 8th segments (* note: not in the 7th as there is the presence of one of the lateral hearts ), a pair of stout pulsating branches send blood to the pharyngeal nephridia, oesophagus and gizzard.
II. Ventral vessel:
It is large and principally a distributing blood vessel.
Position :
The ventral blood vessel runs mid-ventrally below the alimentary canal and above the nerve cord throughout the length of the body.
Anatomy :
Its walls are thin and non-contractile and without valves.
Blood flows posteriorly.
* Note: Major differences between dorsal blood vessels and ventral blood vessels.
Distribution of blood by Ventral blood vessels:
It is principally a distributing vessel.
1. By ventro-tegumentary vessels, it supplies to
a. integumentary nephridia,
b. body wall,
c. septa and
d. reproductive organs
of each segment.
2. This ventro-tegumentary vessel, behind the 13th segment sends a small branch to septal nephridia called a septo-nephridial branch.
3. To the intestine ( gut wall ), called ventro-intestinal vessel.
III. Lateral Oesophageal vessels:
These are two vessels.
Position :
Lying on either ventrolateral side of the gut, from the anterior end to the 13th segment.
As collecting vessel :
In each segment, these receive a pair of ventral-tegumentary vessels, collecting blood from the
a. body wall,
b.septa,
c.nephridia and
d. reproductive organs.
As distributing blood vessel
Posteriorly some of its blood passes to the supra-oesophageal vessel through a pair of anterior loops in segments 10 and 11 through several ring vessels running via the walls of the stomach.
The rest of the blood flows backwards into sub-neural vessels.
IV. Sub-neural vessel:
It is a collecting vessel.
Anatomy:
It is a slender vessel formed by the union of two lateral oesophageal vessels
Position:
In the mid-ventral region below the nerve cord from the 14th segment onwards.
Blood flow :
From in front backwards.
Collects blood:
Receives blood from the
a. ventral nerve cord
b. ventral body wall
in each segment through small branches.
Pours blood :
into the dorsal vessel via a pair of commissurals.
V. Supra-oesophageal blood vessel:
It is a collecting vessel.
Anatomy:
Short and thin-walled.
Position:
Mid-dorsally placed, above the stomach in segments 9 - 13th.
Connected to:
a. Lateral oesophageal vessel by two pairs of anterior loops.
b. Ventral vessel by two pairs of lateral-oesophageal hearts.
It collects :
From the stomach, gizzard and lateral oesophageal ( through anterior loops ).
Distribute to :
Pumps it to ventral vessels through latero-oesophageal hearts.
Lateral or Transverse Blood Vessels:
Longitudinal blood vessels are connected directly or indirectly through numerous segmentally placed transverse or lateral blood vessels.
They can be subdivided as
1. Lateral blood vessels of Anterior region ( first 13 segments) :
a. Hearts :
Position: In segments 7, 9, 12 and 13
Anatomy :
A pair of large, thick, muscular and rhythmically contractile vertical vessels.
blood flow :
Pump blood from the dorsal vessel to the ventral vessel. Flow in the opposite direction is prevented by valves.
Valves:Present.
Lateral hearts :
Hearts of the 7 and 9 segment connect dorsal and ventral vessels only.
Latero-oesophageal hearts: Hearts of 12 and 13 segments connect both dorsal and Supra-oesophageal vessels with ventral vessels.
b. Anterior loops :
Position :
10th and 12th segment.
Anatomy :
Pair of thin-walled, non-pulsatile, non-muscular and loop-like broad vessels.
Valves: Absent.
Blood flow :
Convey blood from lateral-oesophageal into Supra-oesophageal vessels.
c. Ring vessels: Circular vessels of a typical nature.
Position: In the stomach, within the muscular coat.
Number :
About 12 vessels per segment. Through these vessels, blood of the lateral-oesophageal reaches the supra-esophageal.
d. Ventro-tegumentary vessels :
A pair is given by a ventral vessel.
position :
In each segment.
Blood flow :
To the body wall, septa nephridia and reproductive organs of the same segment.
2. Lateral vessels of the intestinal region ( behind the 13th segment)
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| T.S of body, Pheretima through intestinal region showing arrangement of blood vessels |
a.Ventro-tegumentary vessels :
Position: In the posterior region of the segment, in the intestinal region, the ventral vessel gives off ventro-tegumentary vessels on either side.
Branches :
It branches off to supply septal nephridia by sending a delicate septo-nephridial branch, which runs upward along the anterior face of the septum.
It also pierces the septum to enter the segment behind and supply its body wall and integumentary nephridia.
b. Commissural vessels:
Position :
Behind the 13th segment, a pair in each segment along the posterior face of the septum, on either side.
Conveys :
- Blood from sub-neural vessel to dorsal vessel.
- Gives a small branch to the intestinal Plexus called the septo-intestinal branch.
It receives :
Several branches from septa, nephridia and body wall.
c. Dorso-intestinal :
Position:
In the region of the intestine, in each segment, two pairs of these vessels are present.
Conveys :
These convey blood from the intestinal wall to the dorsal vessel.
d. Ventro-intestinal :
Single median vessel.
Position :
In each segment of the intestine.
Function :
Carries blood from ventral vessel to intestinal wall.
III. Intestinal Plexuses :
The wall of the intestine contains many blood capillaries arranged in two networks:external plexus and internal plexus.
a. External plexus:
Position: Lies on the surface of the gut.
It receives:
Blood from ventral vessels through ventro-intestinals and septo-intestinals.
Conveys :
The received blood is conveyed to the internal plexus.
b. Internal plexus :
Position :
Present between circular muscles and enteric epithelium.
Convey :
Along with the absorbed nutrients, the internal plexus passes blood to the dorsal vessel through the dorsal intestinal tract.
Circulation of Blood :
The flow of blood is from behind forwards in the dorsal vessel and from in front backwards in the ventral and sub-neural vessels.
The ventral vessel supplies blood to the body wall septa, nephridia reproductive organs etc. through the ventro-tegumentary vessels and to the gut through the ventro-intestinal vessels. This blood is collected by the lateral oesophageal and sub-neural vessels.
In the first 13 segments, blood from the gut, body wall, nephridia, septa, gonads etc. is collected by lateral oesophageal vessels and anterior loops.
From the gizzard and stomach, blood is collected by supra-oesophageal which eventually passes its blood to the ventral vessel through lateral-oesophageal hearts.
In the intestinal region, the dorsal vessels collect blood from the gut wall through the dorso-intestinal and from the sub-neural through the commissural vessels which also receive blood from the body wall, septa, nephridia etc.
From the gut wall, the dorso-intestinal vessels carry blood to the dorsal vessel. Thus, the dorsal vessel collects vessels in the intestinal region whereas it distributes vessels in the first 13 segments supplying blood to the gut and to the ventral vessel through the heart.
Thus in earthworms, the circulation of blood occurs.
By the circulation of blood, the digested food absorbed through the intestinal wall is distributed to different parts of the body whereas, CO2 and nitrogenous wastes are carried to nephridia, skin and coelomic fluid for elimination.
Blood glands:
Not much is known about this gland but in segments 4, 5, and 6 are found small red-coloured follicular bodies which lie above the pharyngeal mass and are connected with pharyngeal or salivary glands.
Each gland consists mass of loose cells surrounded by a capsule with a syncytial wall.
Function :
They aid in the manufacture of blood corpuscles and haemoglobin.
( Note: Many researchers regard these glands as excretory. )
Related Important Questions:
Question:
Give an account of the blood vascular system in the anterior 13 segments of Pheretima.
Answer :
In the anterior 13 segments the blood vascular system consists of the following vessels :
1. Dorsal vessel:
It is the largest vessel running mid-dorsally above the alimentary canal throughout the body but it functions differently according to the segments.
Anteriorly before the 13th segment it acts as a distributing vessel. It sends out all the collected blood from the posterior region of the body into the ventral vessel through the heart.
Extending up to the cerebral ganglion in the middle line of the 3rd segment, dorsal vessel trifurcates giving blood to the pharyngeal bulb and roof of the buccal chamber and in each of the 3rd, 4th, 5th, 6th and 8th segments, it sends stout pulsating branches to the pharyngeal nephridia, oesophagus and gizzard.
Ventral vessel:
It is a large vessel running mid-ventrally below the alimentary canal and above the nerve cord throughout the body.
It is principally a distributing vessel.
Before the 13th segment and throughout each segment, a pair of ventral tegumentary vessels supply blood to integumentary nephridia, body wall, septa and reproductive organs.
Lateral oesophageal vessels :
These are two vessels, one on the either ventrolateral side of the gut, running from the anterior end up to the 13th segment.
These receive a pair of ventro-tegumentary vessels in each segment, collecting blood from the body wall, septa, nephridia and reproductive organs.
From here some of its blood passes to the Supra- oesophageal vessel through a pair of anterior loops in the 10th and 11th segment and through several ring vessels in the wall of the stomach.
The rest of the blood flows backwards into a sub-neural vessel which forms after the 13th segment.
Sub-neural vessel:
Sub-neutral vessel starts from the 14th segment but it is also a collecting vessel and through commissural, it pours blood to the dorsal vessel which moves from backward to front and is distributed again.
Supra-oesophageal vessel :
It is a short thin-walled collecting vessel lying mid-dorsally above the stomach from segment 9 to 13.
It is connected to the lateral-oesophageal vessel through two pairs of anterior loops and to the ventral vessel through two pairs of latero-oesophageal hearts. It collects blood from the stomach, gizzard and lateral oesophageal ( through anterior loops ) and pumps it into ventral vessels through lateral oesophageal hearts.
Hearts :
A pair of large, thick, muscular and rhythmically contractile vertical vessels called hearts are present in each of the segments 7th, 9th, 12th and 13th. They pump blood from the dorsal to the ventral vessel. They have valves.
Lateral hearts: Those of the 7th and 9th segments are called lateral hearts which connect dorsal and ventral vessels only.
Latero-oesophageal hearts:
Those of the 12th and 13th segments connect both dorsal and supra-oesophageal vessels with ventral vessels and are called latero-oesophageal hearts.
Anterior loops:
In the 10th and 11th segments are present a pair of thin-walled, non-pulsatile, non-muscular, loop-like broad vessels called anterior loops. They are valveless and convey blood from lateral- oesophageals into Supra-oesophageal vessels.
Ring vessels:
These are circular vessels of the stomach through which the blood of lateral oesophageals reaches the supra-oesophageal.
Ventro-tegumentary vessels:
The ventral vessel gives a pair of ventro-tegumentary vessels in each segment to the body wall, septa, nephridia and reproductive organs of the segment.
Question:
Give an account of the blood vascular system in the segments behind 13 segments of Pheretima.
Answer :
In the segments behind 13 segment the blood vascular system of Pheretima consists of the following vessels:
1. Dorsal vessel:
It is the largest blood vessel of the body running above the elementary canal throughout the body it has a pair of valves in each segment in front of the septum. The blood flows through it from backwards to forward.
Behind the 13th segment, the dorsal vessel is a collecting vessel receiving blood through two pairs of dorso-intestinal vessels in each segment from the intestine and a pair of commissural vessels from sub-neural vessels in each segment.
2. Ventral vessel:
Behind the 13th segment, the ventral vessel supplies blood through :
- A pair of ventro-tegumentary vessels to integumentary nephridia, body wall, septa and reproductive organs.
- This ventro-tegumentary vessel sends a small branch called the septo-nephridial branch supplying the septal nephridia.
- Also, the ventral vessel gives off a median ventro-intestinal vessel to the intestine.
Sub-neural vessel :
The two lateral oesophageal vessels on either side of the cut from the 14th segment merge to form this slender vessel which runs immediately beneath the nerve cord in the mid-ventral position. It runs posteriorly till the end. The flow of blood is from front to back.
Through a pair of small branches, the sub-neural vessel receives blood from the ventral nerve cord and ventral body wall in each segment.
Via a pair of commissurals, it pours blood into the dorsal vessel in each segment.
Intestinal plexuses :
The wall of the intestine contains many blood capillaries arranged in networks called plexuses. They are :
External plexus: Lies on the surface of the gut and receives blood from ventral vessels through ventro-intestinals and septo-intestinals.
From there it passes on to the internal plexus situated between circular muscles and enteric epithelium. The blood along with absorbed nutrients is passed on to the dorsal vessel the orso-intestinal.
Multiple Choice Questions (MCQs):
1. The blood vascular system of earthworm is :
a. open type
b. closed type
c. portal type
d. none
Answer : ( b ) closed type
2. Earthworm contain blood pigment:
a. haemoglobin
b. haemocyanin
c. haematin
d. melanin
Answer : ( a ) haemoglobin
3. Flow of blood in the dorsal blood vessel of Pheretima is :
a. from in front backwards
b. from behind forward
c. in both direction
d. none of these
Answer : ( b ) from behind forward
4. Which of the following blood vessels is considered the true heart in earthworm :
a. dorsal blood vessel
b. ventral blood vessel
c. supra- oesophageal blood vessel
d. sub-neural blood vessel
Answer : ( a ) dorsal blood vessel
5. The blood vessel having a valve in Pheretima is :
a. dorsal
b. ventral
c. sub-neural
d. Supra-oesophageal
Answer : ( a ) dorsal
6. Blood glands of earthworm take part in :
a. formation of blood corpuscles
b. formation of phagocytes
c. maintenance of blood volume
d. maintenance of blood circulation
Answer : ( a ) formation of blood corpuscles
7. The smallest longitudinal blood vessel in earthworm is :
a. dorsal blood vessel
b. ventral blood vessel
c. supra- oesophageal blood vessel
d. sub-neural blood vessel
Answer : ( c ) supra-oesophageal blood vessel
8. Which of the following is the character of the dorsal blood vessels of earthworms?
a. collecting in the whole body
b. collecting in first 13 segments
c. distributing the whole body
d. distributing in the first 13 segments
Answer : ( d ) distributing in the first 13 segments.
9. Lateral and latero-oesophageal hearts in Pheretima are situated in the following segments :
a. 7, 9, 10, 12
b. 7, 9, 12, 13
c. 7, 8, 12, 13
d. 7, 9, 11, 12
Answer : ( b ) 7, 9, 12, 13
10. The blood of earthworm is red because :
a. haemoglobin is present in blood cells
b. haemoglobin is dissolved in plasma
c. both
d. none
Answer : ( b )
11. Earthworms have :
a. four pairs of heart
b.no heart
c. 2 pairs of heart
d. Blue Blood
Answer : ( a ) four pairs of heart
12. The lateral hearts of earthworms are present in :
a. 5th and 8th segment
b. 7th and 9th segment
c. 10th and 12th segment
d. 13th and 14th segment
Answer : ( b ) 7th and 9th segment
13. The flow of blood from the lateral hearts present in 7 and 9 segments of Pheretima is from the:
a. dorsal to the ventral blood vessel
b. ventral to the dorsal blood vessel
c. dorsal to lateral oesophageal blood vessel
d. lateral oesophageal to the dorsal blood vessel
Answer : ( a ) dorsal to the ventral blood vessel.
14. Lateral oesophageal hearts in earthworms connect :
a. dorsal and ventral vessel
b. dorsal and supra-oesophageal vessel with ventral vessel
c. dorsal and sub-neural vessel
d. lateral oesophageal and supra-oesophageal vessels
Answer : ( b )
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