Circulation in Pheretima
Earthworm has a system of vessels or tubes in which red blood circulates which transports oxygen, carbon dioxide, digested food and nitrogenous waste to and from the parts of the body. A transportation system, in which blood circulates always in tubes, is called a closed circulatory system. The circulatory system comprises: blood, blood vessel, hearts and blood glands.
The blood is composed of a fluid plasma and colourless corpuscles which can be comparable to the leucocytes of vertebrates. Haemoglobin occurs dissolved in the plasma which gives red colour to the blood.
2. Blood vessels
The arrangement of blood vessels is very complicated. The arrangement in the first 13 segment is different from the arrangement of blood vessels in rest of the body. Thus it is easy to describe the circulatory system under two headings:
I. Arrangement of the blood vessels behind the thirteenth segments.
In this region there are three main longitudinal vessels namely.
(i) Dorsal vessel
(ii) Ventral vessel
(iii) Subneural vessel.
(i) Dorsal vessel: Dorsal vessel is the largest of the three longitudinal vessels. It runs throughout the length of the body in the mid-dorsal line between the alimentary canal and the body-wall. Its walls are thick and muscular. It shows rhythmical peristaltic contractions from behind to the front. The backward flow of blood cannot take place as in this vessel, a pair of forwardly directed valves are found in its lumen in front of each septum. During contraction-movement, the two flaps of valve close down, thus, preventing the backward flow of blood. In this region dorsal vessel receives blood and thus serves only a collecting vessel but does not distribute the blood. In each segment, it receives two pairs of dorso-intestinal vessels collecting blood from two pairs of transverse vessels encircling the intestine. It also receives, in each segment, a pair of commissural vessels one on each side, which are ventrally connected with the subneural vessel. Commissural vessels run along the posterior face of the septum, one on either side and receive capillaries from the body-wall and nephridia.
(ii) Ventral vessel: It is a large vessel that runs mid-ventrally below the alimentary canal and above the nerve cord from one end of the body to another. Its walls are thin and non-contractile and the valves are altogether absent. The blood flows through it posteriorly.
The ventral vessel is principally a distributing vessel. It supplies blood, in each segment through a pair of ventro-tegumentary vessels to the integumentary nephridia, body wall, septa and reproductive organs. Behind the 13th segment each ventro-tegumentry vessel sends as in all branch, a septo-nephridial branch, supplying the septal nephridia. Besides these, the ventral vessel, in each segment behind the 13 gives off a median ventro- intestinal vessel to the intestine.
(iii) Subneural Blood Vessel: In the intestinal region blood from the body wall and nerve cord is collected by another longitudinal vessel, the sub-neural. It lies in the mid-ventral line beneath the nerve cord and extends from the posterior end of the animal to the fourteenth segment in front. It communicates in each segment with the dorsal vessel through a pair of commissural vessels. Blood flows backwards in this slender vessel.
II. Arrangement of the blood vessels behind the first thirteenth segments.
The first thirteenth segments have the following vesels.
(i) Dorsal vessel: It becomes the distributing vessel at this region and extends up to the cerebral ganglion. It gives off branches which supply the anterior regions of the body. It sends back blood to the ventral vessel through the “hearts”.
(ii) Supra oesophageal vessel: A very short vessel extending between segments 9th and 13th. It is a collecting vessel and colletcs blood from gizzard and stomach.
(iii) Lateral oesophageal vessel: These vessels have been formed by the bifurcation of the subneural vessel at the 14th segment. These are collecting vessels and continue anteriorly along the lateral sides of the oesophagus. Through a pair of branches in each segment they collect blood from lateral regions of gut, body wall, septum and seminal vesicle.
(iv) Ventral vessel: It extends anteriorly up to the 2nd segment and is a supplying vessel. It supplies blood to ventral body wall, septal nephridia and reproductive organs. It gives a pair of ventrotegumentary vessels in each segment.
3. Hearts: There are four pairs of hearts, one pair each in 7th. 9th, 12th and 13th segment. They
pump blood from the dorsal to the ventral vessel. They possess valves to cheek
backward flow of blood. Since the hearts of 7th and 9th segments connect the dorsal
and ventral vessels, they are called lateral hearts. They possess four valves each. The
hearts of 12th and 13th segments communicate both the dorsal and supra-oesophageal vessels with the ventral vessel and are designated as the lateral oesophageal hearts. They possess three
Ring Vessels: There are about twelve circular vessels called ring vessels present per segment within the wall of the stomach through which blood from the lateral oesophageal
is passed to supraoesophageal vessel. Blood from the supraoesophageal is sent into the
ventral vessel through the hearts present in segments 12 and 13. These two pairs of hearts
are distinguished from the other two pairs as latero-oesophageal hearts. Rhythmical
contractions of the dorsal blood vessel and four pairs of hearts maintain circulation of blood
in these vessels.
4. Blood glands
There are several small red follicular bodies just above the pharyngeal mass in each of the 4th. 5th and 6th segment. These are called blood glands and are connected to the salivary glands. The follicles have syncytial wall enclosing capsule containing mass of loose cells. These glands manufacture blood corpuscles and haemoglobin and also supposed to be excretory in nature.
Circulation of Blood
The flow of blood is from behind forwards in the dorsal vessel and from in front backwards in the ventral and subneural 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 ventro-intestinal vessels. This blood is collected by the lateral oesophageal and subneural vessels. In the first 13 segments blood from gut, body wall, nephridia, septa, gonads etc. is collected by lateral oesophageal vessels and anterior loops. From 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 collects blood from gut wall through dorso-intestinal and from subneural 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 is collecting vessels in the intestinal region where as it is distributing vessel in the first 13 segments
supplies blood to the gut and to the ventral vessel through hearts.