Hydras (scientific name: Hydra) are part of the order Cnidaria, a family of animals that include sea anemones and jellyfish. Hydras are small marine organisms that have a genetic peculiarity: they are immortal. These polyps do not have a brain but are made up of stem cells capable of constantly reconstituting themselves. Located in the animal's head, these self-regenerating cells allow the animal to live forever. In the 18th century, the Swedish naturalist Carl von Linné named the animal after the hydra, a mythical seven-headed serpent from Greek mythology whose heads grew back as they were cut off.