Circulatory system is well developed and very complex due to the two modes of respiration. It consists of
(i) Pericardium
(ii) Heart
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(iii) Arteries
(iv) Sinuses and
(v) Veins.
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1. Pericardium:
It is thin walled chamber lying dorsally on the left side of the body whorl. It extends anteriorly upto the stomach and the digestive gland. It communicates with posterior renal chamber through a reno-pericardial aperture. Pericardium originates from the coelom and enclosed heart.
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1. Heart:
It is two chambered-an auricle and a ventricle.
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Auricle:
It is a thin walled contractile, triangular sac situated on the dorsal side of the pericardium.
It receives blood through three main veins
(a) Efferent ctenidial vein from the gill
(b) Efferent renal vein from the posterior renal chamber and
(c) Pulmonary vein from the pulmonary sac.
On the ventral side auricle opens into ventricle through auriculoventricular aperture, guarded with two semilunar valves. These valves prevent the backflow of blood into the auricle.
2. Ventricle:
It is a spongy structure with reduced cavity due to a coarse meshwork of muscular strands.
A large artery, aortic trunk arises from the ventricle and its opening is also guarded by two semilunar valves. Valves prevent backflow of the blood into the ventricle.
3. Arteries:
Main artery called aortic trunk immediately gives off two branches, anterior cephalic aorta and a posterior visceral aorta.
Cephalic aorta:
Near the base of aorta a thick walled and highly contractile structure, the aortic ampulla is found. Rhythmic contractions of the ampulla help in the circulation of blood.
Immediately beyond the ampulla three arteries arise on outer side
(i) A fine cutaneous artery to the skin
(ii) A thick oesophageal artery to the oesoophagus and
(iii) A stout left pallial artery to the left side of mantle, the left nuchal lobe and the osphradium.
On the inner side, a pericardial artery arises to supply pericardium, then goes to posterior renal chamber. Its renal branch supplies to anterior renal chamber and to a part of genital organs.
Main trunk of cephalic aorta continues and gives a branch to perivisceral sinus and then runs over the oesophagus towards right side.
Numerous small branches arise from it to supply oesophagus wall and the floor of the mantle cavity.
On right side it gives a large branch which further divides into three arteries
(i) A right pallial artery to the right part of mantle
(ii) A right siphonal artery to the right nuchal lobe and
(iii) A penial artery to the penis.
Main trunk further sends
(i) A radular sac artery to the radular sac
(ii) Optic arteries to the eyes and stalk
(iii) Tentacualr arteries to the tentacles and finally
(iv) Pedal arteries to the foot.
Visceral aorta:
It runs posteriorly into the visceral mass and supplies the visceral organs through a series of numerous branches:
These are:
(i) A small pericardial artery to the pericardium, skin and digestive glands
(ii) A large gastric artery to stomach
(iii) Several renal intestinal arteries to the intestine,
(iv) Several renal arteries to the roof of posterior renal chamber
(v) A large artery to the digestive gland and gonads
(vi) A second set of several intestinal arteries to the intestine,
(vii) Several renal arteries to the roof of anterior renal chamber and
(viii) Several arteries to the terminal part of the genital duct.
After giving out these branches the visceral aorta finally terminates in the wall of the rectum.
4. Sinuses and veins:
Blood, supplied by arteries and their branches to various parts of the body collects in small spaces called lacunae.
Some of lacunae join arteries through veins while others join together to form larger spaces called sinuses.
They collectively constitute the haemocoel. Veins carry all the blood from lacunae and sinuses to the auricle of the heart either directly or through ctenidium mantle or kidneys. Main sinuses and their corresponding veins are as follows:
(a) Peri-visceral sinus:
It collects the blood supplied by the cephalic aorta (except pericardial) and some branches of the visceral aorta. From this sinus blood from it is drained into afferent ctenidial vein through a number of branches.
(b) Peri-intestinal sinus:
It lies along the coils of the intestine. It collects blood from the viscera, digestive gland and greater part of the reproductive system.
It is drained into renal chambers through afferent renal sinus and afferent renal vein. Blood comes from anterior renal chamber and goes into posterior renal chamber. Finally blood carries to the auricle of the heart by an efferent renal vein.
(c) Branchio-renal sinus:
It lies to the right of anterior renal chamber. It collects blood from the renal chambers and drained into afferent ctenidial vein. This vein receives blood from rectal wall and pericardial sinus also.
(d) Pulmonary sinus:
It lies in the wall of the pulmonary sac. It collects blood from the perivisceral sinus through afferent pulmonary veins. After aeration blood is carried to the auricle of the heart through efferent pulmonary vein.
Blood and its course of circulation:
The respiratory pigment dissolved in plasma of blood is haemocyanin. It turns blue when in oxygenated state. It contains phagocytic cells also.
Ventricle of the heart pumps blood into all body parts through the branches of visceral aorta. From all parts it is then collected into perivisceral and peri-intestinal sinuses.
Blood then undergoes oxygenation and filteration in gills, pulmonary sacs and kidney and is received back into the auricle through major veins. It then goes into ventricle and is pumped again to all parts of the body.