Body segmentation first appeared in the phylum Annelida (annulus meaning "little ring"). Organisms of the Kingdom Animalia having three primary embryonic cell layers, i.e., the ectoderm, mesoderm, and endoderm, from which different tissues and organs of the animal's body differentiate, are known as triploblastic. Examples are frogs, flatworms, and humans, as well as all multicellular animals except sponges and coelenterates.

The notochord is a mesodermally derived rod-like structure that is formed on the dorsal surface of the body during embryonic development in some animals. Those animals with notochords are called chordates. Animals that do not have notochords are called non-chordates.

Bilateral Symmetry divides the organism's body into identical halves of left and right through a vertical plane known as the sagittal plane or medial plane.