The Circulatory System
Blood is pumped from the heart into the arterial trunks of the Limulus (shown in red).
Two large venous trunks, the cardinals (shown in pink) collect the blood from the anterior, lateral and posterior parts of the body and conduct it to the book gills.
Reoxygenated by the book gills, the blood then flows into the pericardial membrane and back to the heart.
Illustration (modified) by Carl Shuster