难以根除的祸害, [动]水螅

Hydra \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.