Hydra

[hahy-druh]

Hydra, Greece: see Ídhra.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

hydra, common name for freshwater organisms in the phylum Cnidaria, which includes jellyfish, sea anemones, and corals. Hydras are widely distributed in lakes, ponds, and sluggish streams. They are small, cylindrical, solitary organisms, the largest reaching about 1 in. (2.5 cm) in length. They attach themselves temporarily to water plants or submerged objects, using an adhesive pedal disk at the anal end. The simple body consists of an outer layer of epidermis, a middle noncellular layer of mesoglea, and an inner layer of gastrodermis lining the simple gastrovascular cavity, where the final stages of food digestion occur. A whorl of tentacles surrounds the mouth. Hydras feed on small plankton organisms, especially microcrustaceans, stunning them with stinging cells (nematocysts) in the tentacles. Hydras reproduce asexually by budding and sexually by means of gonadal cells formed on the sides of the body. The green hydra, Chlorhydra viridissima, contains green algae living symbiotically in its gastrodermal cells. The gray and brown hydras belong to the genus Pelmatohydra. Several species of hydra also occur in American waters. Hydras are classified in the phylum Cnidaria, class Hydrazoa.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

Hydra

Hydra, in astronomy, southern constellation lying S of Corvus, Crater, Virgo, Leo, and Cancer. It is a long, slender, winding constellation traditionally depicted as a snake and known also as the Water Monster or Sea Serpent. The only bright star in the constellation is Alphard (Alpha Hydrae); it is a double star, but the two components can be detected only with a telescope. Hydra reaches its highest point in the evening sky in the spring.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

Hydra

Hydra, in Greek mythology, many-headed water serpent; offspring of Typhon and Echidna. When one of its heads was cut off, two new heads appeared. The second labor of Hercules was to kill the monster. He did so by burning the neck after cutting off each head.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

hydra

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).

Learn more about hydra with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Hydra, Saronic Islands

Hydra (Ύδρα, ) is one of the Saronic Islands of Greece, located in the Aegean Sea between the Saronic Gulf and the Argolic Gulf. It is separated from the Peloponnese by the narrow Hydra Gulf. Hydra is also a municipality and one of the few provinces in Greece to have two municipalities---which includes the island of Dokos (pop. 43)---yet fewer than five communes. In ancient times, the island was known as Hydrea (Υδρέα), which was a reference to the springs on the island.

An island dependent upon tourism, Athenians comprise a sizeable segment of Hydra's visitors. Hydra is served by high speed hydrofoils and catamarans from Piraeus plus daily island tour boats. Besides garbage trucks, motor vehicles are not permitted on the island, which leaves the bulk of public transportation up to horses, donkeys and water taxis. The inhabited area, however, is so compact that most people walk everywhere.

There is one main town, known simply as "Hydra port" (pop. 2,526 in 2001). It consists of a crescent-shaped harbor, around which is centered a strand of commercial establishments (restaurants, shops, markets, and galleries), all of which cater to tourists and locals (Hydriots). Steep stone streets lead up and outwards from the harbor area. Most of the local residences, as well as the hostelries on the island are located on these streets. Other small villages or hamlets on the island include Mandraki (pop. 33), Kamini, Vlychos (28), Palomitha (18), Episkopi (23), and Molos (2).

Although the island's name is derived from ancient springs, it is now almost dry. While Hydra previously had wells, these were drained by seismic activity around the mid-20th century. Today, the island's water is imported by boat from the Greek mainland. The dominant geographic feature of Hydra is its rocky hillsides, which are bare, save for the occasional farmhouse and six Orthodox monasteries.

Hydra's harbor, market, and buildings were perhaps an architectural influence for resort developer Charles Fraser to create Harbour Town, the well known village at Sea Pines Resort on Hilton Head Island, South Carolina, USA.

Books about or set on Hydra

Clouds over Hydra, Charles Young (1996)
The Colossus of Maroussi, Henry Miller (1941)
Fugitive Pieces, Anne Michaels (1996)
Hydra and the Bananas of Leonard Cohen, Roger Green (2003)
Lighthousekeeping, Jeannette Winterson (2006)
Rhubarbs from a Rock, David Fagan (2003)
The Riders, Tim Winton (1996)
Rien ne va plus (The Sleepwalker), Margarita Karapanou (1994)

Movies filmed on Hydra

The Blue Villa (Un Bruit Qui Rend Fou) (1995)
Boat Trip (2002)
Boy on a Dolphin (1957)
Fugitive Pieces (2007)
Incense for the Damned (1970)
Girl in Black (Greece 1956)
Out of the Shadows (1988)
Phaedra (1962)

Theater on Hydra

Hydrama Theater and Arts Center hosts performances, drama and dance workshops for the local community and courses in ancient Greek theater for international participants.