in fabulous history, a serpent in the marsh of Lerna, in Peloponnesus, represented by the poets with many heads, one of which being cut off, another immediately succeeded in its place, unless the wound was instantly cauterized. Hercules attacked this monster; and having caused Iolaus to hew down wood for flaming brands, as he cut off the heads he applied the brands to the wounds, by which means he destroyed the hydra.

This hydra with many heads is said to have been only a multitude of serpents, which infested the marshes of Lerna near Mycene, and which seemed to multiply as they were destroyed. Hercules, with the assistance of his companions, cleared the country of them, by burning the reeds in which they lodged.

in Astronomy, a southern constellation, consisting of a number of stars, imagined to represent a water serpent. The stars in Hydra, in Ptolemy's catalogue, are twenty-seven; in Tycho's, nineteen; in Helvelius's, thirty-one.

in Zoology, a genus of the order of zoophyta, belonging to the class of vermes. See HELMINTHOLOGY Index.