Subject: Why do hydras not kill theirselves if their 'arms' touche theirselves?

I' m a student and I asked my biology teacher, why the nettel cells of hydras don't 
start if a hydra touch the "arms" of another hydra or of a spercial fish. He didn't know 
the answer, I searched for it in the net, but I found nothing. So I hope, you can help me.
It isn't an important question, but it would be great to know it. How do a hydra feel and 
know wheter the "thing" that touches its arms is food or another hydra?

Re: Why do hydras not kill theirselves if their 'arms' touche theirselves?