Hydra is a scientific name. Hydra is a genus of fresh water animals and belongs to the phylum cnidaria and the class hydrozoa.

Hydra was also a serpent like water animal in Greek mythology.

The origin of the name comes from the genus hydra an animal with radial symmetry

and the ability to regenerate limbs similar to the greek monster that you're referring to. It regenerates cells at a remarkable pace to maintain survival like how it never dies from father time just like the Genus Hydra not the Marvel terrorist organization. That name came from Lernaen Hydra.

According to Wikipedia, it is of the class "Hydrozoa": http://en.wikipedia.org/wiki/Hydra_(genus)

Some animals, such as those in the genus Hydra, don't actually have a brain per se, making it difficult to answer.