Blood Vascular System or circulatory system of Herdmania is well developed and greatly modified. It includes : (i) Heart and pericardium, (ii) blood vessels, and (iii) blood.
Heart and pericardium
The pericardium is a noncontractile, elongated and transparent tube about 7 cm long and 3 mm wide, running obliquely below the right gonad. It is closed at both the ends and filled with a colourless pericardial fluid with corpuscles similar to those of blood. Its thick wall is made of connective tissue contains blood sinuses and is internally lined by squamous epithelium.
The heart is enclosed within the pericardium and attached to its wall along its entire length by a thin mesentery-like connective tissue flap. It is a cylindrical highly contractile and thin-walled structure with striated muscles. It is formed by an infolding of the pericardium. 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.
Blood vessels
Blood vascular system is elaborate. While the major blood vessels have definite walls, the smaller ones lack them. The larger blood vessels and their branches are as follows :
- Ventral aorta :- The ventral or subendostylar vessel is the larges, 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 the ventral side of the test, and then bifurcates into an anterior and a posterior hypobranchial branch, running first below the entire length of the endostyle. Both, the branches give off several paired transverse vessels (40 to 56 pairs) to the wall of branchial sac between rows of stigmata, and numerous fine vessels to the endostyle and mantle. The anterior branch joins two circular vessels at the base of branchial siphon—(i) a peripharyngeal vessel that runs below the peripharyngeal groove, and ii) a subtentacular vessel that runs below the bases of branchial tentacles. The latter sends a tentacular branch in each tentacle and 6-8 siphonal vessels into the. branchial siphon. The posterior branch of ventral aorta sends a minor vessel to oesophageal area.
- Dorsal aorta :- It is a stout vessel lying mid-dorsally, just above dorsal lamina in the dorsal wall of branchial sac. It is not connected to the heart but communicates with the ventral vessel through 5-7 pairs of transverse vessels of the branchial sac, and the circular peripharyngeal and subtentacular vessels. From its middle portion, a neural vessel is given into the neural complex. The anterior end of dorsal aorta also sends a small branch to dorsal tubercle and 6-8 siphonal vessels into the mantle of branchial siphon.
- Branchio-visceral vessel :- It is a very short vessel in posterior continuation of dorsal aorta. It immediately divides into two branches. The right branch or right oesophageal vessel is short and supplies the right liver lobe and right side of oesophagus. The left branch or ventro-intestinal vessel is stout, long and supply blood to left side of oesophagus, stomach, intestine, rectum, left gonad and left liver lobe.
- Cardio-visceral vessel :- It arises from the dorsal end of the heart and supplies blood to several organs through different branches. Immediately near origin, it sends a right hepatic vessel to right liver lobe and an oesophageo-test vessel to oesophagus and test, (i) The main dorsal branch sends a test vessel, a left oesophageal and a right gonadial to respective organs. It further extends to form a circular subtentacular vessel at the bases of atrial tentacles and 6-8 siphonal vessels into the wall of atrial siphon, (ii) The main middle branch or left gonadial vessel passes obliquely into the left gonad, (iii) 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.
Blood
Blood is slightly reddish, transparent and hypertonic to sea water. It contains a few colourless amoeboid leucocytes, 6 or 7 types of coloured corpuscles with or without nucleus and without or with one, few or several vacuoles and the nephrocytes having vacuoles and colloidal cytoplasm. The pigment found in corpuscles may be orange, yellowish-brown or yellowish-green, but never red.
Most tunicates can extract the rare metal vanadium from sea water and store it in green blood corpuscles called vanadocytes. But its oxygen absorbing power is very low and real function is uncertain. Some tunicates, including Herdmania, can not extract vanadium from sea water. In ascidians capillaries are absent but sinuses are present. Therefore, there is no distinction between blood and tissue fluids due to free intermixing.
Course of circulation
No valves are present in the heart of Herdmania to regulate the flow of blood which is maintained by peristaltic waves and the small pear-shaped body. However, the ascidian heart 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, its oxygenated blood, collected through ventral aorta from branchial sac and test, is pumped into the cardiovisceral vessel and distributed to the various parts of the body (test and viscera).
The deoxygenated blood from viscera is collected by the branchio-visceral vessel which passes it to the dorsal aorta from where it goes into the transverse branchial vessels to become oxygenated once again to undergo a fresh cycle. During reversal of heart beat in dorso-ventral direction, the deoxygenated blood collected through cardio-visceral vessel from viscera, is pumped into ventral aorta and distributed into transverse branchial, peripharyngeal, subtentacular and test vessels. The blood now oxygenated, is collected by dorsal aorta and distributed once again to viscera through branchiovisceral vessel. Deoxygenated blood from viscera is collected by cardio-visceral vessel and brought back to the heart to restart the cycle.