Body plan and symmetry
Reference: Symmetry in biology
Almost everything that moves under its own power is bilaterally symmetrical with a front end — and that decision was taken once, very early, and has effectively never been revisited.
Three plans
- Bilateral — a left and a right, a front and a back. Vertebrates, insects, molluscs, worms. Directed movement puts the sense organs at the leading end, and a head is the result. Every creature in this catalogue that pursues anything is built this way.
- Radial — jellyfish, sea anemones, adult starfish. No front. Suits a creature that waits, drifts, or meets its environment equally from all sides. Starfish are the odd case: their larvae are bilateral and the radial adult is built on top of that.
- None — sponges, most fungi, and much of the plant kingdom, which grows to fit rather than to a fixed layout.
Why it never gets revisited
Body plan is settled earliest in development, so everything built later depends on it. A change to the plan is a change to the foundation of an already-standing structure, and the creature does not survive to be judged on the merits.
This is the same shape of argument as the four-limb constraint, and it is worth being clear that it is an explanation of the constraint, not independent evidence for how the constraint arose. Deep developmental entrenchment is what a frozen accident looks like; it is also what a fixed specification would look like.