(About the primaeval worm, our ancestor of 590 million years ago.) Any animal that moves, in the sense of covering the ground from A to B rather than just sitting in one place and waving its arms or pumping water through itself, is likely to need a specialized front end. It might as well have a name, so lets call it a head. The head hits novelty first. It makes sense to take in food at the end that encounters it first, and to concentrate the sense organs there too – eyes perhaps, some kind of feelers, organs of taste and smell. Then the main concentration of nervous tissue – the brain – had best be near the sense organs, and near the action at the front end, where the food-catching apparatus is. So we can define the head end as the leading end, the one with the mouth, the main sense organ and the brain, if there is one. Another good idea is to void wastes somewhere near the back end, far from the mouth, to avoid re-imbibing what has just been passed out. (Dawkins, 2004),