Up and down in the story of evolution

(About the primaeval worm, our ancestor of 590 million years ago.) Why is there a dorsal side and a ventral side? The argument is similar (as with the fore and aft asymmetry), and the one applies to starfish just as much as to worms. Gravity being what it is, there are lots of inevitable differences between up and down. Down is where the sea bottom is, down is where the friction is, up is where the sunlight comes from, up is the direction from which things fall on you. It is unlikely that dangers will threaten equally from below and above, and in any case those dangers are likely to be qualitatively different. So our primitive worm should have a specialised upper or ‘dorsal’ side and a specialised ‘ventral’ or lower side, rather than simply not caring which side faces the sea bottom and which side faces the sky. (Dawkins, 2004),