The number of Hox genes grows as the animal gets more complex, but the genetic sequences are very conservative; mammals use the same set of regulatory genes for head formation as do arthropods.
The most primitive animals, sponges, have at least one Hox gene. It is posited that when Hox genes reached a critical number, a huge variety of body plans arose. But the set of interelated sequences established then became fixed by virtue of its interrelatedness. Minor changes have taken care of diversification needs since the Cambrian.
Fractal from the Complexity Exhibit, Hooper Museum.