Widdershins

yup. Am thinking a distributed nervous system - autonomous functions like balance, breathing and so forth as subbrains all reporting to (and overridable by) the central brain which, like the most important organs is situated deep inside the body and protected by bone.  The heads then can house what amounts to the audio and visual cortexes - normally a large part of a mammal's brain, and by that method that can scale with the number of heads the creature has.

as a more disturbing thought though - imagine that only 80% of each head's brain is used for local processing, but the intelligence of the creature is a single mind spread over the remaining 20% of each brain. That would mean every time you cut off a head, the mind loses memories, but grows back smarter than before...

There is a bit of purely spiritual defining to do there though. Lopping off a head, while likely not ideal for the health of anybeing, would mean the brain also regrows and.... well, brains don't work that way. They form uniquely from use and you don't get memories from a new brain. 

 But hey, Dragons can pull off an awful lot with the excuse of "It works because magic."

Likely, I say that most hydras aren't particularly bright and just need to bite at whatever goes by. Like most sea life.

 There is also the lesser known trait... that if hydras grow heads... if too many heads crop up, then the body can't support them and it falls apart trying to. That's why I say "self-pruning" is important for a hydra. That, while two-heads-in-place-of-one is thematic for a story and for heroics..... that can't biologically work even with magic. That growing heads works like tree rings and is a sign of the near limitless age and with age, comes the knowledge that anymore heads means competition for food and a strain on both the body and environment. 

Biologically & Theorhetically proven Funcitonal Self-Maulings! Whoo! Logic!

Indeed.

I choose to look at it more like how Google and other petascale providers operate. A storage location fails? well, that's what redundancy is for; buy and power on two more, and rebuild the lost data.  how about losing compute? yeah,that's expected; the system requeues the tasks that were not completed, and again, you buy and power up two more to replace what was lost.  lose sensor data? replace and upgrade...

learning how to make huge distributed systems took the big operators years of experimentation, but then, evolution has had millions of years to iteratively design the systems it uses, and, in many cases, we just reinvent wheels that nature already perfected long before we came out of the trees.