Hydras live in water and are close relatives of jellyfish. They are also known as freshwater polyps. Hydras are hollow, catch prey in the water with their arms and digest it in the cavity of their torso.

The hydra of Greek mythology is a many-headed monster. If one head is cut off, two new heads grow back.

Why did this small aquatic animal get the name of such a fearsome monster? Does it rise out of the water like its mythical namesake to tear apart herds of cattle? Does it have many heads? Probably not. But the biological Hydra is just as good at regrowing cut-off body parts as the Greek mythical Hydra. If a living Hydra is cut in two with a knife, two complete Hydras are formed from the two halves. That is legendary!