What Is The Difference Between Hydrocephalus And Hydrancephaly?

I suppose, put simply Hydrocephalus is 'water on the brain' while Hydranencephaly is 'water IN rhe brain'
The latter is a rare disorder where one or both cephalic hemispheres are absent and replaced by sacs filled with cerebro-spinal fluid (CSF), while the former is more common and only the ventricles of the brain are filled with CSF

Hydro is water compressing the brain, as noted on the brain.
Hydra is the existence of water, where brain matter is non existent.  Most patients do however have a brain stem allowing them to function normally.  The prognosis is that a slim percent will see their first birthday.  The majority of babies born with this disorder, expire.