\Hy"dra\, n.; pl. E. {Hydras}, L. {Hydr[ae]}. [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. (Zo["o]l.) Any small fresh-water hydroid of the genus
   {Hydra}, usually found attached to sticks, stones, etc.,
   by a basal sucker.
Note: 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.