Animals that fall into the category of vertebrates can be classified most simply as animals with a back bone. While this is the general understanding of what a vertebrate is, not all vertebrates have a definitive back bone, and the very first organisms to fall into this category only had a notochord which helped develop the nervous system.(3) Other characteristics of vertebrates includes a distinct head, a differentiated brain, and three pairs of sense organs.(3) These sense organs include nasal, optic, and otic which is hearing.(3) Another thing that is unique to vertebrates is their neural crest cells. These cells help in the development of the skull and jaw, and many people believe that it is one of the main characteristics of vertebrates.(2)
Vertebrates are not the biggest group of organisms, but they are the most accessible animals for us to study and find especially considering humans are vertebrates. Many of these animals live in much more human-friendly places.(2) The there are 5 main groups of vertebrates which are fishes, amphibians, birds, reptiles, and mammals. The types of fishes include Agnatha, Chondrichthyes, and Osteichthyes, and they are the types of vertebrates that will be focused on here.(1)
Sources:
1."Classification of Living Things: Classes of Vertebrates." Classification of Living Things: Classes of Vertebrates. Accessed February 08, 2017. http://anthro.palomar.edu/animal/animal_4.htm.
2."Introduction to the Vertebrates." Introduction to Vertebrates. Accessed February 08, 2017. http://www.ucmp.berkeley.edu/vertebrates/vertintro.html.
3. Rafferty, John P. "5 Vertebrate Groups." Encyclopædia Britannica. Accessed February 08, 2017. https://www.britannica.com/list/5-vertebrate-groups.