With the exception of Cephalopods, members of the Mollusca phylum have an open circulatory system. Having an open circulatory system means that blood is not restricted to circulating within the blood vessels. An open circulatory system pumps blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the pumped blood.
