A class of transcriptional control genes called Hox genes directs the organization of the major animal body plans, and these genes are strongly homologous across the animal kingdom The relationships between the Eumetazoa and more basal clades (Ctenophora, Porifera, and Placozoa) are still being debated.

Although animals range in complexity from simple sea sponges to human beings, most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan.

animal kingdom — the presence of differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific functions. Hox gene — (also, homeobox gene) master control gene that can turn on or off large numbers of other genes during embryogenesis. Eumetazoa — group of animals with true differentiated tissues. body plan — morphology or defining shape of an organism.