Hundreds of octopus species have three hearts: one for their circulatory system and two for their gills.
Squids have hundreds of species and a broad range of sizes, just like their cousins the octopi, yet they all have three hearts.
The majority of earthworm species are very small, but a few varieties, known as giant earthworms, can reach lengths of up to 10 feet.
Hagfish have four hearts in addition to being most known for their ability to create enormous volumes of slime.
The cuttlefish is closely related to the octopus and the squid, two other creatures having numerous hearts.
It is commonly known that they can survive tough environments and suffer serious wounds, and that is one reason why they are so difficult to kill.
Frogs circulate blood around the horse's circulatory system, therefore they serve as a heart. So, horses have five hearts.
When a heart transplant is needed, the original heart is sometimes not completely removed. The new heart is grafted into the old, giving the patient two.