Cephalopod molluscs, like octopuses and squids, have a closed circulatory system with blood pumped under pressure flowing within vessels. Bivalve molluscs, like mussels and oysters, have an open circulatory system (also known as lacunar circulatory system) where blood flows under low pressure since it falls in cavities of the body and does not only circulate within blood vessels. Molluscs with closed circulatory systems are larger, agile and can actively move; molluscs with open circulatory systems are smaller, slow and some are practically sessile.