Down a little way below the surface, where there is still plenty of food, and not too many predators, protected by your increasingly armoured shell, you grow longer and longer legs to help you move more and more quickly to evade the few creatures trying to eat you.

Your frontmost legs become longer and more specialised to manipulation. In fact, as you’re now quicker and stronger than a lot of smaller creatures, you could take a leap sideways and start actively looking for other creatures and eating them, rather than just eating the detritus you find?