in Greek legend, the offspring of Typhon and Echidna (according to the early Greek poet Hesiod's Theogeny), a gigantic monster with nine heads (the number varies), the centre one immortal. The monster's haunt was the marshes of Lerna near Argos. The destruction of Hydra was one of the 12 Labours of Heracles, which he accomplished with the assistance of Iolaus. As one head was cut off, two grew in its place; therefore, Iolaus finally burned out the roots with firebrands as soon as Heracles cut off each head. At last Heracles severed the one immortal head from the body and buried it under a heavy rock.The arrows dipped by Heracles in the poisonous blood or gall inflicted fatal wounds, eventually including his own accidental death at the hands of his wife, Deianira (according to Sophocles' tragedy Trachinian Women). In modern English, hydra or hydra-headed can describe a difficult or multifarious situation.

Eggs and sperm appear in separate swellings (gonads) in the outer body layer, and individuals usually have separate sexes. Some species, however, are hermaphroditic (i.e., functional reproductive organs of both sexes occur in the same individual). Eggs are retained in the ovaries and fertilized by sperm from neighbouring individuals. Offspring are eventually released as miniature hydras. Vegetative reproduction by budding is also common. Finger-shaped outpushings of the wall develop mouth and tentacles and finally nip off at the base, forming separate new individuals. Locomotion is by creeping on the adhesive base, or by looping; i.e., tentacles attach to the substrate, the base releases, and the whole body somersaults, enabling the base to attach in a new position.

The genus is represented by about 25 species, which differ chiefly in colour, tentacle length and number, and gonad position and size. All Hydra species feed on other small invertebrate animals such as crustaceans. Hydra is an unusual hydrozoan genus in that its life cycle lacks any trace of a jellyfish stage, and the polyp stage is solitary rather than colonial.

First prominent in the late 15th century under Turkish rule, it became a seafaring centre. In the 17th century the island received an influx of Albanian refugees from the Peloponnese; maritime trade then thrived. After an abortive insurrection against the Turks in 1770, Hydra received Greek refugees, who also concentrated their energies on commercial shipping. In 1821, at the outbreak of the War of Greek Independence, the island's population had risen to 30,000. With neighbouring Spétsai and Psará islands, the Greeks and Albanians of Hydra placed their considerable merchant fleets and fortunes at the disposal of the insurgents, and Hydriote sea captains commanded Greek ships in several successful encounters with the Turkish fleet.

With the advent of steamships, however, the maritime activities of the island declined. Industries now include sponge fishing, cotton weaving, shipbuilding, and international tourism. Ídhra, the chief town, on the north coast, is an artists' and writers' colony and the residence of a metropolitan bishop. Its narrow, rock-cut streets surround a sheltered harbour. Three other small ports on the north coast are Mandrákion, Mólos, and Panayía. Area 19.2 square miles (49.6 square km). Pop. (2001) 2,672.