Definitions:
a minute freshwater coelenterate with a tubular body and a ring of tentacles around the mouth.

a many-headed snake whose heads grew again as they were cut off, eventually killed by hercules.

the largest constellation (the water snake or sea monster), said to represent the beast slain by hercules. its few bright stars are close to the celestial equator.

a thing that is hard to overcome or resist because of its pervasive or enduring quality or its many aspects

used with preceding greek letter or numeral to designate a star in the hydra constellation