Cardiovascular system
The Quahog clam has an open cardiovascular system. This system includes the ventricle, intestine, auricle, and pericardium are all included as part of this system. The pericardium encloses the heart and its major vessels. Blood is pumped out of the heart and into some large vessels. These vessels lead to tissues, and exit out of arteries. The blood then passes over organs, and is absorbed by the organs' cells. Afterward, the blood fills up spaces known as lacunae, from where it passes through the kidney and gills to return to the heart, and the cycle restarts itself.
Take a look at this paper on Quahog Clams!
http://www.extension.org/sites/default/files/Biology%20and%20Culture%20of%20the%20Northern%20Quahog%20Calm.pdf
http://www.extension.org/sites/default/files/Biology%20and%20Culture%20of%20the%20Northern%20Quahog%20Calm.pdf