HYDRA, in Greek legend, the offspring of Typhon and Echidna, a gigantic monster with nine heads (the number is variously given), the centre one being immortal. Its haunt was the marshes of Lerna by Argos. The destruction of this hydra was one of the twelve "labours" of Heracles, which he accom plished with the assistance of Iolaus. Finding that as soon as one head was cut off two grew in its place, they burned out the roots with firebrands, and at last severed the immortal head from the body, and buried it under a rock. The arrows dipped by Heracles in the poisonous blood or gall inflicted fatal wounds. See II. 444.