Many cephalopods (including squids and octopuses) have three hearts, while earthworms have five hearts and hagfish have four. An operation exists in which a surgeon can put an additional heart into a human that can supplement the original.
Octopuses and squids have one primary heart to pump oxygenated blood throughout their bodies and two more operational hearts to pump blood to their gills. Earthworms have five hearts, called aortic arches, one to pump blood through each segment of their bodies. The unique hagfish has between five and 15 pairs of gills and four hearts. One heart acts as the primary pump, and the other four supplement it when needed.