Mythology of the constellation Hydra
Hydra was a nine-headed monster in the form of a long, writhing water snake.
It attacked, killed and ate anyone who ventured near its swampy abode in Lernea.
To slay this dreadful beast was the second of Hercules's twelve labours.
This was an enormously difficult task, since every time Hercules succeeded in
cutting off one of Hydra's heads, two would grow where the one had been.
Added to this, Hera, who had always been resentful of Hercules, sent a giant
crab to nip and bite at his feet as he attempted to slay the monster.
He finally achieved this after instructing his nephew, Iolaus, to burn each
neck stump immediately after he, himself, had severed the head, thus preventing any regrowth.