Anatomy of finfish and shellfish

Unit 13- Mollusca

13.4.5. Circulatory System
The blood vascular system is well developed with a complete separation of venous and arterial blood. It consists of the heart, arteries, veins, and a system of capillaries.
There are three hearts in Sepia, as in all the dibranchiate cephalopods. The systemic or arterial heart lies in the middle of the visceral mass enclosed in the pericardium. It consists of a thick-walled median ventricle, and two thin walled lateral auricles, all spindle-shaped. The ventricle is slightly constricted into two lobes; it supplies arterial blood through a large anterior or cephalic or oral aorta and a small posterior or aboral aorta, to the anterior and posterior regions of the body, respec¬tively. The aortae branch into arteries which lead into a system of capillaries and then into veins.
The venous blood of the head returns by a large, vena cava, which bifurcates in front of the rectum into right and left branch ial vein leading into the so-called branchial harts, laying one at the base of each ctenidium. Each banchial or gill, heart also receives directly a small pallial vein from the mantle and an abdominal abdominal vein from the posterior body region. The unpaired genital and ink sac veins join the right branchial vein. Each branchial heart pumps blood to the ctenidium of its side through an afferent branch/al vein, running through the axis of the gill and giving off branches as it goes. The oxygenated blood of a ctenidium is returned by an efferent branchial vein first to the auricle of that side, and then to the ventricle.
The blood, containing haemocyanin and amoehocytes, is colourless when venous and pale-blue when oxygenated.