A generalized schematic of the cephalopod molluscs circulatory systems (top) and a more anatomically correct view (bottom), showing the well-developed hearts (a ventricle and two branchial hearts) and complex, endothelial-like lined vascular systems (peripheral and branchial). The cephalopod vascular system is considered “closed” with highly efficient hearts pumping blood at pressures similar to those seen in lower vertebrates (adapted from Smith and Boyle [16], Ruppert et al. [12]).