Hox genes in animals are the homeobox genes that control body plans. Animals are segmented. There are maternal effect genes which control the polarity (direction) of the body, segmentation genes which control the polarity of each segment and homeotic selector genes which determine the identity of each segment. All the tissues and organs of the body develop from 2 - 3 primary tissue layers (primary germ layers). In mammals, there are 3: the endoderm, ectoderm and mesoderm. Segments in the embryo (somites) controlled by hox genes develop in the order in which the hox genes are expressed. The hox genes are expressed in the order in which they appear on the chromosome.
Most animals are either radially symmetrical (top and bottom) or show bilateral symmetry (left and right). Sponges are asymmetrical.