Each segment develops two thicker patches on the upper sides. It’s harder to kill and eat you, but you do move slower. In time, the patches of thicker armour extend further and further.

Your simple eyes develop further, by accidental repetition which at first gives you a slightly better view of the world, into larger eyes containing hundreds of simple eyes - which gives you a much improved view on the world[1].

As you get bigger, you need to get oxygen into your tissues, as you’ve become too big to just let it happen passively. Some of your central segments lose their feet and become thinner and thinner to maximise the amount of surface area, and let the oxygen in.

You can wriggle around the world, in and out of the water, eating other wormy creatures. There are fossils showing your tracks which end at a worm’s burrow. But when you’re a predator eating something like that, there are two good ways to go about it, which mean two different types of mouth evolve.