The segmented body plan is useful. You can speed up quite a bit by having long muscles under the skin which pull both against your insides (hydrostatically) but also against your jointed outsides. You get a little faster, and avoid getting eaten.
For some time now, you’ve had cells which are aware of light changes. They help you know if you are hiding under a rock under out in the open, for example. As you get bigger and faster, some of these cells happen to develop in pits in your skin. This is very good because they begin to give you awareness of the direction light is coming from instead of just the general light level. This makes you more aware of your environment and thus more effective, and more likely to reproduce, and so the genes for ‘light cells in pits’ spread. Other possible arrangements are found by random changes, and the genes causing the better ones are more likely to spread through the population. Eventually, the light cells are found in deep, circular pits, giving good quality information about light in a very specific direction. After perhaps as few as several hundred more generations, there is a membrane of fine skin over the light pits, and the pits contain clear fluid. The simple eye has arrived.
At the same time, your segments develop more shape which gives you more speed. Like the ridges on the bottom of a pair of walking boots, those among you with rough surfaces at the bottom of the segments have more grip, and are more successful. Evolution ruthlessly leaves behind the least efficient, until only those with extrusions that look like clawed feet on the sides of the bases of the segments are left.
You are tough, fast, and able to see what is in front of you. In fact, you can now eat small worms, and you certainly do. You are a hunting worm, which preys upon smaller worms - as well as being hunted by larger worms.
You might prefer to concentrate on eating other animals, and stay lightweight and fast, and never mind that some of you are eaten. Alternatively, you might prefer to stiffen up that skin into armour plates, and accept that this will make you slower because it will also make you less vulnerable?
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