The notochord extends from head to tail region and is persistent throughout the life in

The notochord extends from head to tail region and is persistent throughout the life in cephalochordates (e.g., Branchiostoma). In urochordates (e.g., Doliolum), it is present only in the larval tail. In vertebrates (e.g., Exocoetus), the notochord is present during the embryonic period; it is replaced by a vertebral column in the adult. The notochord is absent in non-chordates (e.g., Dentalium, a mollusc).