You stay in the sea, and those flattened, footless, central segments multiply to provide you with more and more oxygen[1], so you can get bigger and bigger.
Staying in the sea, although you can walk on the land for several hours at a time, means relying on a proven source of food. As a heavily armoured crawling hunter though, you will often see the newly evolved jawless fish swimming above you, and maybe some other lifeforms that you could eat too.
If you push off and wiggle hard enough, you might catch some of them too.
Worth a try?
[1] You now have many sections under your middle, each one as flat as possible and as thin skinned as possible to allow as much oxygen through as they can. The end result is a gill structure that looks like the pages of a book. These are called “book gills”.
