Wikipedia
Branchial hearts are myogenic accessory pumps found in coleoid cephalopods that supplement the action of the main, systemic heart. Each consists of a single chamber and they are always paired, being located at the base of the gills. They pump blood through the gills via the afferent branchial veins. Since they only circulate venous blood, branchial hearts function under predominantly anaerobic conditions. Branchial hearts also appear to be involved in hemocyanin synthesis.
Each branchial heart is directly connected to a branchial heart appendage or pericardial gland. The action of the branchial hearts is necessary for the production of primary urine in these appendages via pressure filtration. Branchial hearts may have evolved from the pericardial glands of nautiloids, such as those still found in modern nautiluses.