The nervous system includes the nerves and the brain, and the nervous system carries information from various parts of the body to the brain. Not all living organisms have even a rudimentary nervous system--single-celled creatures, for instance, have no nervous system. The nervous system developed first in prehistoric chordates and then became more and more complex as evolution produced more and more complex creatures. We can see what the earliest nervous systems must have been like in more primitive chordata such as the Amphioxus. The nervous system can then be traced up through more complex organisms to its highest manifestation in the human being. Information about the outside world and about the inner workings of the body speeds to and from the brain through nerves, which are bundles of the long, tubelike extensions of nerve cells. Information is conveyed through them by impulses fired along their length. Nerve impulses travel only in one direction. As an example, if information from one part of the body is transmitted to the brain through one nerve, orders from the brain cannot travel back along the same nerve but must be carried by a different nerve. This shows how complex the system has become, but the system began in a much simpler form.
The chordates are of the phylum Chordata and include the well known vertebrates (fishes, amphibians, reptiles, birds, mammals). Vertebrates and hagfishes together comprise the taxon Craniata, while the remaining chordates are the tunicates (Urochordata), lancelets (Cephalochordata), and perhaps some odd extinct groups. With few exceptions, chordates are active animals with bilaterally symmetric bodies and are longitudinally differentiated into head, trunk, and tail. The most distinctive morphological features of chordates are the notochord, nerve cord, and visceral clefts and arches. The chordates are well represented in marine, freshwater and terrestrial habitats from the Equat...