..an organism must either armor itself ( like a tree
      or a clam ) and "hope for the best," or else develop
      methods of getting out  of harm's way  and  into the
      better neighborhoods in its vicinity.  If you follow
      this latter course,   you  are  confronted  with the
      primordial problem that every agent must continually
      solve:

                  Now what do I do?            

      In order  to solve this problem,  you need a nervous
      system, to control your activities in time and space.
      The juvenile  sea squirt  wanders  through  the  sea
      searching  for a suitable  rock or  hunk of coral to
      cling to and  make its home for life.  For this task,
      it has a  rudimentary  nervous system. When it finds
      its spot  and takes root, it doesn't  need its brain
      anymore, so it eats it! ( It's  rather like  getting
      tenure.)