Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.12.Circulatory system
The circulatory system is well developed and of the open types. It consists of (1) the heart and pericardium (2) arteries (3) sinuses, (4) veins and (5) blood. Capillaries are absent.

Fig. 16. Unio. Circulatory system

Heart and pericardium
Near the mid-dorsal line ant just in front of the posterior adductor muscle, there is a thin walled triangular chamber, the pericardium, which encloses the heart. The pericardium, lined by an epithelium and filled with fluid, represents a portion of coelom and communicates with the supra-branchial chamber through the kidneys.
The heart is highly contractile and 3-chambered. It consists of a single ventricle and two auricles, lying one on either side of the ventricle. The auricles are transparent, thin walled roughly triangular and distensible to a great extent. The receive blood returning from the gills and mantle. Each auricle is attached to the pericardial wall by a broad base and opens into the ventricle by an auriculo-ventricular aperture, having valves opening towards the ventricle. The single median ventricle is a large, thick-walled, and muscular and tubular chamber, wrapped around the rectum, and pumping blood to the body. The muscular heart beats from 20 to 100 times per minute, so that movement of the blood is made possible.
The circulation is rather slow, but it seems to be adequate for such a slow-moving, sedentary animal.
Arteries
The ventricle pumps blood, both forward and backward, through two main arteries, known as the anterior aorta and posterior aorta, respectively. The anterior aorta runs anteriorly, dorsal to the intestine, and supplies an anterior pallial artery to the mantle, pedal artery to the foot and visceral artery to the visceral mass. The visceral artery supplies the stomach, digestive gland, intestine and gonad through the gastric, hepatic, Intestinal and genital branches, respectively. The posterior aorta runs posteriorly, ventral to the intestine and supplies the rectum, mantle, pericardium, nephridia, etc.
Sinuses
The arteries break up into a network of smaller branches in all the tissues of the body emptying into irregular cavities, the blood sinuses or lacunae, which lack the epithelial lining of true blood vessels and connect directly with veins. The freshwater mussel, therefore, has an open circulatory system.
Veins
The venous blood from various parts of the body is collected by several sinuses and smaller veins, whence it is finally collected by a large longitudinal vein, the vena cava which lies beneath the pericardium in between the kidneys. The vena cava supplies all its blood, through afferent renal veins. to the kidneys, where the nitrogenous waste is eliminated from it. From kidneys, the blood is collected by efferent ;! veins, and passes to the gill of its side for oxygenation throu longitudinal afferent branchial vein, which gives off a branb to each gill filament. From gills the oxygenated blood is returned to the auricles by efferent branchial veins. The vena cava also sends some venous blood directly to the auricles.
The mantle also serves as an important respiratory organ and it sends aerated blood directly to the auricles, through pallial veins, without having passed through the gills or kidneys.
Blood
The blood consists of colourless plasma without haemoglobin but with haemocyanin and with numerous white amotheid cells, or leucocytes, floating in it. There are no red cells as found in the blood of man and other vertebrate animals. The blood distributes oxygen and nutriment to all parts of the body and transfers CO₂ and other waste products of metabolism to the gills, mantle and kidneys.

Fig. 17. Unio. Diagrammatic representation of course of circulation of blood

6. Course of circulation. The course of circulation can b summarized as follows—