/huy"dreuh/, n., pl. hydras, hydrae /-dree/ for 1-3, gen. hydrae /-dree/ for 4.
1. (often cap.) Class. Myth. a water or marsh serpent with nine heads, each of which, if cut off, grew back as two; Hercules killed this serpent by cauterizing the necks as he cut off the heads.
2. any freshwater polyp of the genus Hydra and related genera, having a cylindrical body with a ring of tentacles surrounding the mouth, and usually living attached to rocks, plants, etc., but also capable of detaching and floating in the water.
3. a persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.
4. (cap.) Astron. the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.
[1325-75; < L < Gk hýdra water serpent (r. ME ydre < MF < L); see OTTER]

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Any of 20–30 species of freshwater cnidarians (genus Hydra).

The polyp-type body is a thin, usually translucent tube that measures up to slightly more than 1 in. (25 mm) long. Food is ingested and wastes are ejected from the open, tentacled end. Individuals are usually hermaphroditic (see hermaphroditism). Reproduction by budding is also common. Species differ in colour, tentacle length and number, and gonad position and size. All hydras feed on other small invertebrates (e.g., crustaceans).

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      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.

▪ hydrozoan genus
 genus of invertebrate freshwater animals of the class Hydrozoa (phylum Cnidaria). The body of such an organism consists of a thin, usually translucent tube that measures up to about 30 millimetres (1.2 inches) long but is capable of great contraction. The body wall is comprised of two layers of cells separated by a thin, structureless layer of connective tissue called the mesoglea and the enteron, a cavity containing intestinal organs. The lower end of the body is closed, and an opening at the upper end both ingests food and ejects residue. Around this opening is a circlet of 4 to about 25 tentacles.

      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.

Modern Greek  Ídhra  
 island of the Saronic group in the Aegean Sea, Attica nomós (department), Greece. It lies just off the eastern tip of the Argolís peninsula of the Peloponnese and has a maximum length, northeast-southwest, of 13 miles (21 km). The highest point, Mount Ere, is 1,936 feet (590 metres). Once quite wooded and well watered, as its Turkish name, Çamlıza (“Place of Pines”), shows, it is now denuded and dry, with almost no arable land. Water is collected from rain in cisterns and is also shipped from the mainland.

      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.

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Universalium. 2010.

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