Chordata (chordates)

Typical Examples
Mammals, fish, reptiles, birds, amphibians

Main features:
Segmented coelomates with a notochord; possess a dorsal nerve cord, pharyngeal slits, and a tail at some stage of life; in vertebrates, the notochord is replaced during development by the spinal column.

Description of Chordates
Phylum Chordata: it includes humans and other vertebrates. However, not all chordates are vertebrates.

Four basic features of all Chordates:

(1) Dorsal Nerve Cord - a bundle of nerve fibers which runs down the "back". It connects the brain with the lateral muscles and other organs.Chordates derive their name from one of their synapomorphies, or derived features indicating their common ancestry. This is the notochord , a semi-flexible rod running along the length of the animal. In those chordates which lack bone, muscles work against the notochord to move the animal.

(2) Notochord - cartilaginous rod running underneath, and supporting, the nerve cord.All chordates have a notochord at some stage in their lives, but in some (such as tunicates) the notochord is lost in the adult, whereas in others (such as the vertebrates) the notochord is present in the embryo, but in later stages is largely replaced and surrounded by the vertebrae, or backbones.  The notochord runs beneath the dorsal nerve cord, which is another chordate feature. This is in contrast to organisms such as annelids and arthropods, in which the main nerve cord is ventral. The chordate nerve cord is hollow, with pairs of nerves branching from it at intervals and running to the muscles. The anterior (forward) end of the nerve cord is often enlarged into a brain.

(3) Pharyngeal Slits - a series of openings that connect the inside of the throat to the outside of the "neck". These are often, but not always, used as gills.Pharyngeal slits are a third chordate feature; these are openings between the pharynx, or throat, and the outside. They have been modified extensively in the course of evolution. In primitive chordates, these slits are used to filter food particles from the water. In fishes and some amphibians, the slits bear gills and are used for gas exchange. In most land- living chordates, the "gill slits" are present only in embryonic stages; you had pharyngeal slits at one time. The slits are supported by gill arches, which have also been highly modified in various groups of vertebrates.

(4) Postanal Tail - an extension of the body past the anal opening.Have a post-anal tail, or extension of the notochord and nerve cord past the anus. This feature is also lost in the adult stages of many chordates, such as frogs and people.

In the case of humans and many other vertebrates, these features may only be present in the embryo.