Basically there are two types of animals: animals, and animals that have no brains; they are called plants. They don't need a nervous system because they don't move actively, they don't pull up their roots and run in a forest fire! Anything that moves actively requires a nervous system; otherwise it would come to a quick death.
Actually, after while, finding the ideas is the easy part. Sorting them through and turning them into stories, now, that's the hard work.