Phyla that branch off early, such as sponges and cnidarians, have simple body plans with relatively few Hox loci.
Within phyla, there is no correspondence between the number of Hox genes and complexity
Hox loci are similar in structure and DNA sequence and are grouped in clusters. Hence, they are assumed to have arisen through duplication. When a new Hox gene appears within a lineage, most of the descendant taxa have a homologous Hoxlocus.
In vertebrates, the Hox clusters themselves appear to be duplicated several times.
Within phyla, there is no correspondence between the number of Hoxgenes and complexity