Important types of circulatory system in herdmania are given below:
The circulatory system is well developed and closed type. It includes (1) heart and pericardium (2) blood vessels and (3) Blood
(1) Heart and Pericardium:
It is a non contractile, elongated and transparent tube about 7.0 cm long and 3.0 mm wide running obliquely below the right gonad.
It is closed at both the ends and is filled with a colourless pericardial fluid with corpuscles similar to those of blood.
Its wall is thick, made of connective tissue and contains blood sinuses and is internally lined by squamous epithelium.
The heart is enclosed within the pericardium and is attached to its wall along its entire length by a thin mesentery like connective tissue flap.
It is formed by an infolding of the pericardium but it is highly contractile, thin walled and contains striated muscles.
Both ends of heart are open. There are no valves but a pear shaped body present midway in the pericardial body, probably regulates the flow of blood in the heart.
(2) Blood vessels :
Blood vascular system is elaborate. The major blood vessels have definite walls, but the smaller ones lack them.
(a) Ventral aorta:
The ventral or subendostylar vessel is the largest vessel of the body arising from the ventral end of the heart.
At its point of origin it first gives out a stout ventral test vessel supplying blood to the ventral side of the test.
It then bifurcates into an anterior and a posterior hypobranchial branch running below the entire length of endostyle.
Both the branches give off 40-56 pairs of transverse vessels to wall of branchial sac and several fine vessels to the endostyle and mantle.
The anterior branch joins two circular vessels at the base of branchial siphon- (a) a peripharyngeal vessel and (b) a sub-tentacular vessel.
Later branch sends tentacular branches into each tentacle and 6-8 siphonal vessels into the branchial siphon. The posterior branch supplies a minor branch to oesophagus area.
(b) Dorsal aorta:
It is a stout vessel lying mid-dorsally on the dorsal wall of branchial sac. It is not connected to the heart, but communicates ventral aorta through 5-7 pairs of transverse vessels of the branchial sac and the circular peripharyngeal and subtentacular vessels.
Dorsal aorta supplies branches to neural complex dorsal tubercle and 6-8 siphonal vessels into the mantle of branchial siphon.
(c) Branchio-visceral vessel:
It arises from posterior part of the dorsal aorta and immediately divides into two branches. First branch, right oesophageal vessel is short and supplies the right liver lobe and right side of oesophgus.
Second branch, the ventro-intestinal vessel is stout and long and supplies blood to left side of oesophgus, stomach, intestine rectum, left gonad and left liver lobe.
(d) Cardio-visceral vessel:
It arises from the dorsal end of the heart and supplies blood to several organs. Immediately near origin it sends a right hepatic vessel to right liver lobe and an oesophageo-test vessel to oesophagus and test.
A main dorsal branch sends a test vessel, a left oesophageal and a right gonadial vessels to supply blood to respective organs.
It also gives off 6-8 siphonal vessels into the wall of atrial siphon. A middle branch or left gonadial vessel passes obliquely into the left gonad.
The main ventral branch soon divides into a dorso- intestinal vessel to left liver lobe, stomach and intestine, a gastric vessel to stomach and a dorsal test vessel to test.
(3) Blood :
Blood is slightly reddish, transparent and hypertonic to sea water. It contains a few colourless amoeboid leucocytes 6 to 7 types of coloured corpuscles with or without nucleus and the nephrocytes having vacuoles and colloidal cytoplasm.
Pigment found in corpuscles may be orange, yellowish-brown or yellowish-green but never red and doesn’t take part in respiration.
Course of circulation:
In Herdmania there is no value in the heart and the flow of blood is maintained by peristaltic waves.
The heart of ascidians is unique in the animal kingdom for changing the direction of flow of blood through it by reverse peristalsis at regular intervals. The arteries and veins change their roles when reversal of flow of blood occurs periodically.
When the heart-beats ventro-dorsally it pumps oxygenated blood into the cardio visceral vessel which is distributed to the various parts of the body.
The de-oxygenated blood from viscera is collected by the branchio-visceral vessel and passes through dorsal aorta to finally reach into the transverse branchial vessels for oxygenation.
During reversal phase, the de-oxygenated blood collected through cardio-visceral vessel from viscera is pumped into ventral aorta and is distributed into transverse branchial peripharyngeal, subtantacular and test vessels.
The blood now get oxygenated and is collected by dorsal aorta and distributed once again to viscera through branchio-visceral vessel.
Deoxygenated blood from viscera is collected by cardio-visceral vessel and brought back to the heart to restart the cycle.