All organisms that are found in the chordata phylum of animals have a dorsal nerve cord at some point in their lives. The dorsal nerve cord is an embryonic feature of chordatas. There are three chordata subphylums: vertebrata, urochordata and cephalochordata.
In vertebrate organisms, the dorsal nerve cord turns into the brain and spinal column. Vertebrate organisms are characterized by a backbone composed of bone vertebrates that protect the spinal column. The vertebrata subphylum is made up of eight classes of organisms; these include agnatha, placodermi, chondrichthyes, osteichthyes and amphibia. The remaining classes are reptilia, aves and mammalia. Examples of vertebrate organisms are lampreyes, sharks, lungfish, frogs and snakes. birds and humans.
The placodermi class of organisms contains the first jawed, bony fish, which are now extinct. These organisms were the first vertebrates to exhibit two pairs of appendages. These appendages were the precursors to vertebrates' fins, wings, arms and legs. Organisms that compose the urochordata subphylum do not have vertebrae and only display the dorsal nerve cord during their juvenile stages of life. During adulthood, the dorsal nerve cord degenerates and disappears. An example of an urochordata organism is the sea squirt. Organisms that make up the cephalochordata subphylum are also invertebrates. However, these organisms retain a dorsal nerve cord throughout their lives. An example of a cephalochordata organism is the lancelet.