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The heart lies in the mid‐dorsal region of the body, close to the hinge line of the shell, in a space called the pericardial cavity, which surrounds the heart dorsally and a portion of the intestine ventrally. The wall of the cavity, a thin, translucent layer, is called the pericardium. The heart consists of a single, muscular ventricle and two thin‐walled auricles. Haemolymph – a colourless fluid of haemocytes and plasma (cell‐free haemolymph) – flows from the auricles into the ventricle, which contracts to drive it into a single vessel, the anterior aorta. The aorta divides into many arteries, the most important of which are the pallial arteries from the posterior aorta, which supply the mantle with haemolymph, and the visceral arteries (gastro‐intestinal, hepatic and terminal) from the anterior aorta, which supply the stomach, intestine, shell muscles and foot (Figure 2.13). While a posterior aorta is present in Guekensia spp., it is absent in Mytilus spp. The arteries break up into a network of vessels in all tissues and then join to form veins, which empty into three extensive spaces, the pallial, pedal and median ventral sinuses. The circulatory system is therefore an open system with haemolymph in the sinuses bathing the tissues directly. One consequence of an open circulatory system is that the animal is continually exposed to fluctuations in environmental factors, such as temperature, salinity and contaminants. From the sinuses, haemolymph is carried to the kidneys for purification. In Mytilus, some of the haemolymph from the kidney network enters the gills, discharging into the afferent gill vein, which gives off a branch to each gill filament, descending on one side and ascending on the other. The ascending vessels join to form an efferent gill vein that passes back to the kidneys. The haemolymph from the kidneys returns to the auricles of the heart. In other bivalves (e.g. Pecten spp.), haemolymph from the gills does not return to the kidney but flows directly from the gills to the heart (Figure 2.13). In all bivalves, there are well‐developed circulatory pathways through the mantle, which therefore serves as an additional site of oxygenation. See Field (1922) for a very detailed description of the arterial and venous systems in M. edulis.

Figure 2.13 Circulatory system of a typical bivalve. The shaded areas indicate the route of oxygenated haemolymph. While the bivalve heart has two auricles, only one of these is illustrated. Source:

From Pechenik (2010). Reproduced with permission from the McGraw‐Hill Companies.

Haemolymph plays a number of important roles in bivalve physiology. These include gas exchange, osmoregulation (see Chapter 7), nutrient distribution, waste elimination and internal defence (see Chapter 11). Because haemolymph constitutes 40–60% of the fresh tissue weight, it also serves as a fluid skeleton, giving temporary rigidity to such organs as the labial palps, foot and mantle edges. The haemolymph contains cells called haemocytes, which float in a colourless plasma. Most bivalves lack circulating respiratory pigments, probably because their sedentary lifestyle and large exposed surfaces (for oxygen uptake) preclude the need for such pigments. However, haemocyanin, the typical molluscan respiratory pigment, is found in some protobranch bivalves, while haemoglobin has been reported in several bivalve families (references in Giribet 2008). Haemocytes are not confined to the haemolymph system but move freely out of the sinuses into surrounding connective tissue, the mantle cavity and gut lumen. Therefore, it is not surprising that these cells play an important role in physiological processes such as nutrient digestion and transport, excretion, tissue repair, heavy metal metabolism and internal defence. See Chapter 7 for details on haemocyte types and their functions.