Notochord

The notochord is a rod-like structure that forms on the dorsal (back) side of the body during embryonic development in certain animals. It is derived from the mesoderm. The presence or absence of the notochord divides the animal kingdom into two great groups.

• Animals that form a notochord are called chordates.
• Animals that never form this structure are called non-chordates. This includes all the groups from Porifera up to the echinoderms. …