Some creatures need more than one heart to keep blood flowing throughout their bodies.
Octopus
Because an octopus's blood is very rich in copper, it is viscous, and pumping it requires considerable pressure. To compensate for this, the octopus has evolved three separate hearts.
The systemic (main) heart contains one ventricle and two atria, one atrium on each side of its body. To conserve energy, the systemic heart shuts down when the octopus swims. Only the gill hearts keep working to move oxygen through the gills.
Squid
Like octopuses, squid also have three hearts. They all rest in the mantle cavity – a seawater-filled sac that houses the respiratory and excretory organs.
Hagfish
This eel-like, slimy fish is the only known animal that has a skull but no spine. The hagfish has four hearts, one more than the octopus or squid. One heart is responsible for pumping blood throughout the body. The other three function as accessory hearts.
A hagfish's hearts can keep beating for 36 hours in an oxygen-deprived environment. This remarkable quality explains why hagfish have the lowest blood pressure of any known vertebrate. In addition, they have the highest blood volume relative to body mass of any chordate species. This means hagfish are practically bursting with blood.
Cuttlefish
Unlike most mollusks, cuttlefish have a closed circulatory system. Like squid and octopuses, cuttlefish have three hearts. In the same way, the systemic heart pumps blood throughout the body, while the role of the two gill hearts is to move blood through the cuttlefish's gills.
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Earthworm
Earthworms do not actually have a heart. Instead, they have a heart-like system called the aortic arches. This closed circulatory system carries food, waste and respiratory gases throughout the worm's body.
In all, the aortic arches are made up of five blood vessels. The dorsal vessel runs above its digestive tract and moves blood forward. The other four vessels, including the ventral, subneural and two latero-neural vessels, work to move blood backward.
By the way
For a long time, scientists assumed that dinosaurs also had several hearts to keep blood circulating properly through their enormous bodies. But in the end, modern archaeological and genetic evidence disproved this hypothesis.
