Circulatory System and Circulation

· Circulatory system of earthworms is of closed type as the blood flows in the closed blood vessels.
· It consists of various types of blood vessels, heart, capillaries and Plexi which facilitate collection of the deoxygenated and the distribution of oxygenated blood to all organs and tissues of the body.
· The blood contains the respiratory pigment called haemoglobin which is dissolved in the plasma.
· Only one type of blood corpuscle, the leukocytes are present in the blood which kill the harmful germs.
· No Red Blood Corpuscles (RBC) Present. [BPKIHS 2005; IOM 2007; MOE]
· Circulatory system consists of three types of blood vessels- longitudinal, lateral or transverse blood vessels and intestinal plexus.

I. Longitudinal blood vessels
These function both as collecting and distributing blood vessels. These are five in number.
a. Dorsal blood vessel
· It is the largest blood vessel and lies mid-dorsally above the digestive tract from anterior to the posterior end.
· It has a thick, muscular, pulsatile wall with two valves in each segment, blood flow in this is from backwards to forward direction.
· It collects blood from the segments 14th to last but distributes blood from segments 12th to 1 anteriorly.
· It is connected to the ventral blood vessels by lateral hearts.
· In the first 13 segments, blood is distributed anteriorly to the oesophagus, the pharynx, the gizzard, the stomach etc. In the intestinal region, it collects blood via. dorsal intestinalis.

b. Ventral blood vessel
· It lies below the alimentary canal but above the ventral nerve cord.
· It has thin walls. Valves are absent and it is non-pulsatile.
· It facilitates the flow of blood posteriorly acting as a distribution vessel.
· It supplies blood to the body wall, reproductive organs, intestine.
· It is the main distribution vessel and supplies the blood to all the visceral organs of the body wall.

c. Lateral oesophageal blood vessel
· It is a collecting vessel; two in number, one on either side of the gut from the segments 1 to 13.
· They receive a pair of the ventro-tegumentary vessels in each segment and supply them to segmental organs.
· Through a pair of loops in segments 10 and 11 via many ring vessels, it supplies blood to the supra-oesophageal vessel.

d. Sub-neural blood vessel
· It is a collecting vessel lying beneath the ventral nerve cord and extends from the segment 14th to last.
· It is formed by the union of two lateral oesophageal vessels.
· Each segment receives the blood from a pair of commissural which is a collecting vessel and receive blood from the stomach and gizzard.

e. Supra-oesophageal blood vessel
· It is thin-walled and lies in segments 9-13 above the stomach.
· It is connected to
(a) lateral oesophageal vessel via. anterior loop
(b) to the ventral vessel by lateral oesophageal heart.

II. Lateral (Transverse) blood vessels
· Lateral blood vessels interconnect the longitudinal vessels. These consist of seven types of vessels, some pulsatile and others not. The latter serves to establish connections between various types of blood vessels.

a. Hearts
· Four pairs of hearts [IOM 1999, MOE 2006] are present, a pair each in 7th and 9th (lateral heart) and in 12and 13 (lateral oesophageal).
· These are pulsatile and pump blood from dorsal to ventral valves.
· Valves present in them do not allow reserve flow of blood.

b. Anterior loops
· They connect lateral oesophageal with the supra-oesophageal vessels.
· They have thin, non-muscular walls and do not pulsate rhythmically and are found in the first 13 segments.

c. Ventro-tegumentary blood vessels
· These are lateral vessels of the anterior region and supply the blood to the first 13 segmental organs such as the body wall, septa, nephridia and reproductive organs.

d. Ventro-tegumentary blood vessels
· These are the lateral vessels of the intestinal region behind 13 segments and send a delicate septo-nephridial to septal nephridia and integumentary nephridia.

e. Commissural vessels
· They convey blood from sub-neural to dorsal vessel, supply to intestinal Plexi and also receive vessels from the body wall and septal nephridia.

f. Dorsal intestinalis
· Two pairs of these vessels convey blood from the intestinal wall to the dorsal vessel.

g. Ventro- intestinalis
· Vessels in each segment carry blood from the ventral vessel to the intestinal wall.

III. Intestinal plexus
· Two intestinal plexus are present
(i) external plexus (on the surface of the gut) and
(ii) internal plexus.
· The two are connected by ventro-intestinal and septo-intestinal.
· Intestinal plexus passes on blood, along with absorbed nutrients to dorsal vessels through dorsal intestinalis.

a. Blood glands
· Blood glands are present in segments 4, 5 and 6 above the pharyngeal mass, forming blood cells and haemoglobin.

b. Lymph glands
· Lymph glands occur in segments 26 and those behind it.
· There are two small whitish glands one on either side of the dorsal blood vessel.
· They are believed to generate phagocytic cells.

Excretory or Nephridial system

· Earthworms are both ammonotelic and ureotelic.
· The main excretory organs in them are nephridia [BPKIHS 2000; IOM 2001; KUMET 2008], which consists of slender and coiled tubules, which perform the function of excretion and osmoregulation.
· Nephridia are distributed in all segments except the first two segments.

· According to their location in the body, they are distinguished as septal, pharyngeal and integumentary nephridia.

1. Septal Nephridia
· They show complete organization (holonephridia).
· They are present, attached to the septa behind the 15th segment and each septum bears 40-50 septal nephridia, thus acquiring 80-100 septal nephridia in each segment.

· This nephridium is made up of four components: nephrostome, neck, body and terminal duct.
a. Nephrostome: It is flat; funnel-shaped, ciliated and collects excretory matter from the coelomic fluid and the blood.

b. Neck: The neck is a short, narrow, ciliated tubule that connects the terminal duct with the nephrostome.

c. Body of Nephridium: The body of nephridium is divisible into a short straight lobe and a long twisted lobe which has
(a) one proximal and
(b) distal limb.

d. Terminal duct:
· The terminal short is short, narrow and consists of the ciliated terminal part of the nephridial duct enclosed in a connective tissue matrix.
· A pair of loop-like septal excretory canals in each segment receives the terminal ducts of their side.
· A pair of supra-intestinal excretory ducts are present on the dorsal surface beginning from the segments 15 to the last and receive the segmental (=septal) excretory canal of their side. The former open into the intestine and voids the N2 waste in it.
· This is the reason why septal nephridia are called enteronephric.

Septal nephridium of Earthworm

2. Pharyngeal nephridia
· These are found in pairs in segments 4, 5 and 6 [IOM 2004, 2005] along with blood-glands. They are as large as septal nephridia.
· Funnel and neck are absent.
· Terminal ducts of all nephridia unite to form a common duct (3 pairs). Of these, two open into the buccal and the third in the pharynx.

Pharyngeal and integumentary nephridia of Earthworm

3. Integumentary nephridia
· They are found scattered on the inner surface of the body wall (integument) in each segment except the first two segments.
· In others except for clitellar segments their number is 200-250 and in the clitellar region (14th -16th segments), their number is 2000-2500 (“forests of nephridia”).
· They are the smallest, lack neck and nephrostome [MOE 2063 Falgun] and terminal duct are without cilia.
· They open directly to the outside by nephridiopores hence are exonephric [MOE 2062 Ashad] or ectonephric.

Physiology of excretion
· The urine is acidic and in addition has water and traces of creatinine.
· Chloragogen cells are the sites of urea formation.
· These cells 
(a) deaminate amino acids and
(b) absorb NH3 from coelomic fluid and blood.
· Nephridia function differently in summer, winter and rainy seasons.
· In summer and winter, they reabsorb much more water and conserve it; while in the rainy season, this reabsorption is less and urine is dilute.
· Nephridia of earthworms are osmoregulatory in function.
· Excretory fluids contain 40% urea, 20% ammonia and 40% amino acids and other nitrogenous compounds but no uric acid or urates.

Previous Post Next Post

Main Tags