Mollusks have an open circulatory system since their blood is not always contained in blood vessels. Blood gets pumped by the heart through large blood vessels. The blood exits the arteries and goes into large open spaces where they pass over the cells of different organs.
The blood then enters into the lacunae (a large open space) where it then moves to the kidneys and then to the gills. The gills give the blood more oxygen where it gets sent to the rest of the body.
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