Hydra is a well-known small animal of the order Cnidaria. They are diploblastic animals. They are free-living animals and are known as freshwater polyps. Swiss scientist Abraham Trembley (1700-1784) discovered Hydra. Carolus Linnaeus (1758) named it Hydra after the nine-headed dragon Hydra in Greek mythology. Cutting off one head of the dragon hydra would produce two heads. If any part of it was torn off, a new hydra would be created from that part. That is, hydra has the ability to regenerate. Mahavir Hercules killed the monster Hydra. Dr. Siddiq Publications