Our genes say that we should have 4 limbs and some hair on top of our heads but it doesn’t give coordinates for the every single hair follicle. The same is true for our fingerprints. It follows a pattern, but it can end up in many different ways during fetal development.
There is a constraint to our macroscopic shape. And plants have constraints in the shapes of their parts, but how many copies of those parts they will have and how they will be positioned can change.
Why is that the case?
Because they need their leaves to be in certain shape, but they don’t need their whole body to follow a blueprint.
But why do we have a more or less fixed shape? What do plants and corals have in common that separates them from us?
Its because they are sessile. Thats their commonality.
The thing about centipedes is that they have so many legs that it wouldn’t matter if they had some more. 2 more legs for a centipede is like 5% more hair for a human.
A human with 4 legs would fundamentally have to walk in a different way but for centipedes, the legs are proportional to body length. Mobile organisms have fixed body plans and the only exceptions happen when the length of a section is defined by the number of subsections rather than having a fixed number of them and scaling together. I hope I come up with a satisfactory idea that can be used for the game.
Initially, your sessile species would be no form. But the individual parts can gain symmetries.
Then the species would become mobile. To do this, it would start to distinguish its front and back, and become radially symmetrical.
Then it would start distinguishing its up and down directions and become bileteral.
And lastly it would fix the number of segments it has between its front and back. If you have a heart in one segment, it wouldn’t make sense to copy that heart, so it makes sense for each of the segments to start to specialise. And when you come to something like a giraffe, it no longer makes more bones to make its neck longer, instead it elongates the existing vertebrae.