In mythology and symbolism Hydra is presented as a monstrous serpent, chthonic water beast with seven or nine heads which outgrow if they are cut down. They are often compared with the deltas of rivers because of their multiple arms. Hydra figures multiple vices. It’s heads are the symbol of vices which reappear after being destroyed. The blood of hydra is poisonous: Heracles dips his arrows in it. In case you mix it with waters of rivers, all fishes become inappropriate to eat.
In the Bestiary of Pierre de Beauvais, Hydra kills the crocodile from interior as Jesus Christ destroyed the Hell after having descended there. In the Bestiary of love by Richard de Fournival, hidra is evoked in the part where author tells about the revenge, other men and the fidelity of the lover. In this case Hydra, possessing a lot of heads, is compared to those men who have a lot of women.
In Greek mythology Hydra is a star constellation symbolizing the serpent thrown to the sky by Apollo or Lernaean Hydra defeated by Heracles (as his Second Labor – Hydra was the guardian at the entrance of the Underworld).