The notochord is an elastic rod which extends through Chordate organisms, providing rigid support. In the more derived Chordates, the Vertebrates, the notochord is replaced by the vertebral column and becomes the cartilaginous substance between vertebrae.
The notochord extends the length of the organism, and allows for muscles to attach. This allows the lancelet to swim in fast bursts. Like the lancelet, all Chordate organisms develop a notochord at some point, although they may lose it later in life. Tunicates, for instance, are marine organisms which attach to the bottom of the ocean and filter feed. While the adult form has no need for the notochord, the larval tunicate uses the notochord to swim to potential settling sites. Other species retain the notochord throughout life, and do not grow a vertebral column.
These animals are known as invertebrate chordates. This group includes the lancelets, tunicates, and even some large fish like the sturgeon and coelacanth. In these large fish, the notochord can be as long as they are (around 3-4 feet) and will be really thick, almost half of an inch. These fish use this huge notochord as vertebrates use their spine. The only real difference is what it is made of and how it protects the spinal cord.
In vertebrates, the bony vertebrae grow around the spinal cord, protecting it on all sides. Animals with only a notochord lack this protection, as the spinal cord sits between the notochord and the skin. Animals like the sturgeon and coelacanth have developed armored plates and thick skin to protect their spinal cord and notochord from damage. Vertebrates convert the notochord into the cushioning intervertebral discs, which protect the vertebrae from smashing together. By the time a human is around 4 years old, however, the original notochord has been entirely replaced in the spine with other materials.
The notochord is made of a number of different structural molecules including glycoproteins and resembles cartilage in many ways. When you cut across the notochord, and view the cross section under a microscope, it appears as a series on concentric rings. These different sized rings that surround each other are layers of the notochord, and are made from various structural molecules to give the notochord both strength and elasticity.
The glycoproteins and other structural molecules extend from cells, which are spaced far apart in the notochord. Each of these cells has a large vacuole, which it can pressurize. When pressurized, the cells push against one another and the surrounding structural materials. This creates an extra-rigid notochord, which is necessary for swimming quickly.
The strength of the notochord makes it a very useful structure to attach muscles too. Muscles need places for attachment in order to flex properly. By attaching muscles down the length of the notochord, small invertebrates can use muscles all over their body to swim. Even in the larger fishes which rely on a notochord, it provides enough support for most of the muscles of their body to attach to.
The turgor pressure created by the cells of the notochord makes it extra rigid. While this provided enough support for many organisms, the vertebrates took this body plan one step further with the spine. The spine increases the rigidity of the notochord by being made out of bone and further protects the spinal cord by fully encompassing it.
It has also been found that the notochord serves important signaling functions during normal vertebrate embryogenesis. Notochord secretes proteins that stimulate the formation of organ systems. This process, called organogenesis, starts when the embryo is a hollow ball of cells called the gastrula. This tiny ball has three layers, and a notochord is derived from the middle layer or mesoderm.
Once created, the notochord begins secreting a number of chemical signals which further the process of organogenesis. Eventually, bones form and begin to create the spine. The notochord gets sandwiched between these vertebrae, and provide protection from the bones rubbing or smashing together.