Ecdysozoa

One lucky little prehistoric worm with a gene error expresses a hard protein in its skin, called chitin.  In many other situations this would a problem, making the animal stiff, and slow, and easy prey, and so the gene would die out, but at this point in time it’s just the thing.

Animals with this gene are slightly protected against assault by hunting jellyfish, and after a few generations, selection pressure causes a group to develop with a cuticle - a harder covering than normal ectoderm cells, which aids escape from stinging, grabbing predators.

As you grow, however, this cuticle can’t grow with you, so another mutation rapidly spreads through the population - at intervals, the cuticle is disconnected from the cells producing it, and a new, softer, larger cuticle develops underneath it.  Eventually the old cuticle is sloughed off, and you stretch out inside your new armour.  You are briefly vulnerable while the new cuticle softens, but behaviours such as hiding or burrowing rapidly develop.

At this time, a ‘worm’ is basically a long slim tube comprised of two types of muscle.  Circular ones, which squeeze from side to side and make you lengthen, and a single longitudinal muscle which can shorten the whole length of you.  The first set of muscles contract in groups to extend the front, then the back, then the front again.  It’s simple but it works.  Slowly.  Differentiating the groups more strongly might improve the separation of contractions, and enable a slightly faster gait.

Would you like to develop rings which break up your body into segments?  They will cost a little to make…

I’m smooth
I’m chunky