Sintillian

Sintillians are a short humanoid extraterrestrial species whose hyper-effective hearts render them biologically immortal. They normally possess two hearts, but are able to retain their immortality with a single one. The Sintillian heart is an independent lifeform, capable of moving around after being surgically removed from host, indicating that it might be a symbiotic organism rather than a regular organ. Sintillians are also known to breath a kind of toxic gas which would cause Human lungs to dissolve.