With the exception of the cephalopods, members of the phylum Mollusca have an open circulatory system. The chambered, myogenic heart normally has a pair of posterior auricles draining the gills and an anterior ventricle that pumps the blood through the anterior aorta to the tissue sinuses, excretory organs, and gills. Many gastropods lack a second set of gills, andin these the right auricle is vestigial or absent. The heart is enclosed within the coelomic cavity, which also surrounds part of the intestine. The single aorta branches, and blood is delivered into arterial sinuses, where it directly bathes the tissues. It is collected in a large venous cephalopedal sinus and, after passing through the excretory organs, returns to the gills. The hydrostatic pressure that develops in the blood sinuses of the foot, especially of bivalve mollusks, is used in locomotion. Blood flow into the foot is controlled by valves: as the pressure increases, the foot elongates and anchors into the substratum; muscular contraction then pulls the animal back down to the foot. This type of locomotion is seen most commonly in burrowing species, who move through the substratum almost exclusively by thismeans.

Like the annelids, many mollusks, especially the more sedentary bivalves, set up local feeding and respiratory currents. Fluid movement through the mantle cavity normally depends on muscular pumping through inhalant and exhalant siphons. Within the cavity itself, however, ciliary activity maintains continuous movement across the gill surfaces, collecting food particles and passing them to the mouth.

The cephalopods are more active than other mollusks and consequently have higher metabolic rates and circulatory systems of a higher order of organization. These systems are closed with distinct arteries, veins, and capillaries; the blood (6 percent of body weight) remains distinct from the interstitial fluid (15 percent of body weight). These relative percentages of body weight to blood volume are similar to those of vertebrates and differ markedly from those of species with open circulatory systems, in which hemolymph may constitute 40 to 50 percent of body weight.

The cephalopod heart usually consists of a median ventricle and two auricles. Arterial blood is pumped from the ventricle through anterior and posterior aortas that supply the head and body, respectively. It is passed through the capillary beds of the organs, is collected, and is returned to the heart through a major venous vessel, the vena cava. The vena cava bifurcates(divides into two branches) near the excretory organs, and each branch enters the nephridial sac before passing to the accessory hearts situated at the base of the gills. Veins draining theanterior and posterior mantle and the gonads merge with the branches of the vena cava before reaching the branchial hearts. Contraction of the branchial hearts increases the blood pressure and forces blood through the gill capillaries. The auricles then drain the gills of oxygenated blood.

The blood of most mollusks, including cephalopods, contains hemocyanin, although a few gastropods use hemoglobin. In the cephalopods the pigment unloads at relatively high oxygen pressures, indicating that it is used to transport rather than store oxygen.

Rapid cephalopod locomotion depends almost entirely on water pressure. During inhalation, muscular activity within the mantle wall increases the volume of the mantle cavity and water rushes in. Contraction of the circular mantle muscles closes the edge of the mantle and reduces its volume, forcing the enclosed water through the mobile funnel at high pressure. The force of water leaving the funnel propels the animal in the opposite direction.