Branchial heart

[1] Branchial hearts are always in pairs located at the base of the gills.

[1][2] They pump blood through the gills via the afferent branchial veins.

Since they only circulate venous blood, branchial hearts function under predominantly anaerobic conditions.

[4] The action of the branchial hearts is necessary for the production of primary urine in these appendages via pressure filtration.

[5] Branchial hearts may have evolved from the pericardial glands of nautiloids, such as those still found in modern nautiluses.