"HAIL HYDRA! Cut off a limb and two more shall take its place!"

High-tech global criminal organization in Marvel Comics that frequently opposes Captain America, S.H.I.E.L.D., and the Avengers.

HYDRA was formed during World War II by Baron Von Strucker as a means of extending the Red Skull's organization into the far East. After Germany's defeat HYDRA continued to work toward global domination and the triumph of fascism until Nick Fury and S.H.I.E.L.D. destroyed their island headquarters, whereupon the organization broke apart into several splinter groups.

HYDRA eventually re-formed with an emphasis on terrorism and crime. Silvermane and Viper (as "Madame Hydra", stunning in dark green tights, green-tinted hair, and carrying a whip) have both held the position of Supreme Hydra; Jessica Drew (Spider-Woman) began her career as a HYDRA assassin. Eventually Baron Von Strucker came back and imposed his Nazi ideology on the group once more.

Unlike real terrorists, HYDRA agents are easily spotted in their green uniforms and red-tinted goggles, and are armed with extraterrestrial technology obtained by the group near the end of the war. Occasionally they will kidnap someone and replace him or her with a robotic duplicate known as a Life Model Decoy*. AIM acted as their scientific branch until its leadership was seized by MODOK, who severed AIM's ties with HYDRA.

*My knowledge of this bit of trivia helped settle a debate between Howard Chaykin and Frank Miller once when they were signing at my local comics store. HAIL NAME-DROPPING!

Hydra is a small multicellular animal that lives in freshwater ponds throughout the world. It's tiny, just barely visible with the unaided eye, and sessile, which means it spends most of its life stuck to some piece of dead vegetation or other detritus that floats around in the pond or rests on its bottom.

Tentacles

The hydra owes its name to the shape of its body, which resembles, by a reasonably poetic stretch of the imagination, the Hydra of classical Greek mythology. Most of its body is a thin, stalk-like tube. The base of the body has an organ for adhering to something for stability. At the other end of the body is a head that features six tentacles that branch out from the top of the body tube in radial symmetry. The hydra's mouth, its only opening, is a simple hole located at the center of the part where the tentacles attach to the body.

One interesting ability of the hydra body is regeneration. If any bit of a hydra is cut off, it grows back. That bit can be a tentacle, the whole head part or even most of the stalk.

It's got the nerve

The hydra is extremely simple as animals go. The most important way that it differs from the even simpler and much less interesting sponges is that the hydra has a true nervous system; it is the first organism on the ladder of animal complexity to have one. This evolutionary advance gifts the hydra with the ability to move in complex purposeful ways. It can sense when food gets stuck on its tentacles and the tentacles then move in cooperation to get the food into the mouth hole.

It gets around

Another impressive trick enabled by the nervous system is mobility. While the hydra seldom roams, it can cut loose from its mooring and move to another location when conditions require. It accomplishes that migration by a graceful somersaulting action that is quite like a gymnast doing handsprings, albeit much more slowly.

Hydra sex

There are male hydras and female hydras, but their sexual activities are sure to disappoint you sweaty-palmed, microscope-peering voyeurs. To begin with, hydra reproduce by budding most of the time, which involves no sex at all. Budding happens when some cells on the stalk just start growing into a little baby hydra that eventually pops off on its own. But the sex is even less interesting than that. Males develop spermaries, which generate sperm and spew them out into the water. Females develop ovaries that produce eggs and spew them out into the water. When sperm and egg meet and hit it off, a zygote results and develops into a new hydra. This life cycle is unique among the hydrazoa in that there is no medusa (jellyfish) stage.

Genus: Hydra
Class: Hydrozoa
Phylum: Cnidaria (Coelenterata)
Kingdom: Animalia
Domain: Eurkaryotes

Hydra (sometimes called Pluto III) is one of three known moons of Pluto, and was discovered in June 2005 by the Hubble Space Telescope along with its fellow moon Nix. It is named for the nine-headed serpent charged with guarding a watery entrance to the Underworld in Greek and Roman mythology. Before a name was decided on, Hydra was provisionally dubbed S/2005 P 1.  It is the outermost of Pluto's three moons, and an animation of its place in relation to the other satellites of Pluto can be found here. A glamour shot of the whole family can be viewed here.

Hydra was probably formed as a result of the same large impact astronomers speculate resulted in Charon.

References
http://www.space.com/scienceastronomy/051031_pluto_moons.html
http://www.space.com/scienceastronomy/060621_nix_hydra.html
http://www.boulder.swri.edu/plutonews/
http://en.wikipedia.org/wiki/Pluto_III
http://www.jhuapl.edu/newscenter/pressreleases/2006/060622.asp

Hy"dra (?), n.; pl. E. Hydras (#), L. Hydrae (#). [L. hydra, Gr. "y`dra; akin to "y`dwr water. See Otter the animal, Water.]

1. Class. Myth.

A serpent or monster in the lake or marsh of Lerna, in the Peloponnesus, represented as having many heads, one of which, when cut off, was immediately succeeded by two others, unless the wound was cauterized. It was slain by Hercules. Hence, a terrible monster.

Gorgons, and Hydras, and Chimeras dire. Milton.

2.

Hence: A multifarious evil, or an evil having many sources; not to be overcome by a single effort.

3. Zool.

Any small fresh-water hydroid of the genus Hydra, usually found attached to sticks, stones, etc., by a basal sucker.

⇒ The body is a simple tube, having a mouth at one extremity, surrounded by a circle of tentacles with which it captures its prey. Young hydras bud out from the sides of the older ones, but soon become detached and are then like their parent. Hydras are remarkable for their power of repairing injuries; for if the body be divided in pieces, each piece will grow into a complete hydra, to which fact the name alludes. The zooids or hydranths of marine hydroids are sometimes called hydras.

4. Astron.

A southern constellation of great length lying southerly from Cancer, Leo, and Virgo.

© Webster 1913.