The purpose of a nervous system is to move. There are some sea creatures that have nervous systems when born, but then attach and never move again, digesting their nervous systems at attachment time.
Discussion
Do they coordinate movement with their frens? Travel in groups perhaps?
Or do they recognize and woo their mates?
Even the most simple CNS will have an internal representation of other entities of various categories and keep track of whether each one is Good or Bad in a contextual manner. Good to eat, good to swim alongside, bad to be near bc it will attack me, good to mate with.