. "the water-serpent killed by Hercules near the Lernean Lake , the Hydra , with seven heads; as fast as one of them was cut off two sprang up in its stead; . As identified with Echidna, the mother of Cerberus" . "the water-serpent killed by Hercules near the Lernean Lake , the Hydra , with seven heads; as fast as one of them was cut off two sprang up in its stead; . As identified with Echidna, the mother of Cerberus" . .