Notochord Definition

The notochord permits muscles to connect and expands the length of the organism. The lancelet may now swim in short spurts. All Chordate creatures, including the lancelet, generate a notochord at some time in their lives, albeit they may lose it later.

Tunicates, for example, are marine creatures that cling to the ocean’s bottom and filter food. The notochord is not needed by the adult tunicate, but it is used by the larval tunicate to swim to possible settlement locations. Other animals do not develop a vertebral column and maintain the notochord throughout their lives.

Invertebrate chordates are the name given to these creatures. Lancelets, tunicates, and even huge fish like sturgeon and coelacanth belong to this category. The notochord in these big fish may be as long as they are (about 3-4 feet) and quite thick (almost half an inch).

This massive notochord is used by these fish in the same way that vertebrates use their spine. The only major distinction is the material used and the way it protects the spinal cord. The bone vertebrae develop around the spinal cord in vertebrates, protecting it from all sides.

Because the spinal cord lies between the notochord and the skin, animals with only a notochord lack this protection. To protect their spinal cord and notochord, animals like the sturgeon and coelacanth have developed armoured plates and thick skin.

The notochord is converted by vertebrates into the cushioning intervertebral discs, which keep the vertebrae from colliding. However, by the time a person is about 4 years old, the original notochord in the spine has been completely replaced with different materials.