Pain perception in animals with nerve nets

Are animals with nerve nets, like sea urchins and jelly fish, theoretically able to feel pain? They have output and input organs which would make them "vulnerable" to feel pain but on the other hand, they don’t have brains. I was wondering if they show any signs of "pain" when they are attacked or eaten?

And if they don’t show any signs of pain is there the possibility to measure something that could be interpreted as pain?

Edit:
The word pain is here meant in a way, that it responds to an outer force with behavior which one could interpret as an inner-process that shows discomfort. The important parts are here the words "interpret" and "show signs of".
Pain is here thus meant as a behavioural response that could lead to an interpretation of an inner-process