What is a chordate? How are vertebrates different from chordates?

A chordate is a member of the animal kingdom, and categorized in the phylum Chordata.  Chordates have several characteristics that define them, and set them apart from all other animals.  Some of these traits are only present during embryonic development, but nonetheless, they do serve a crucial purpose within the organism.

Chordates have a notocord, which is essentially a long, flexible rod placed within the organism between dorsal nerve cord and the digestive tract.  The notochord serves many purposes, as it is built out of stiff, fibrous tissues.  Aquatic animals push muscle against this to help them swim, for example.

Chordates also have a dorsal nerve cord, which is a hollow nerve cord.  This dorsal nerve cord, in a human, becomes the spinal cord and brain after the process of embryonic development.

Another characteristic of chordates is the pharyngeal cleft.  The pharynx is the head/neck region of the animal right behind the mouth.  The pharyngeal cleft is a cleft, or pouch like structure in the pharynx region.  The form the pharyngeal cleft takes, and the function it carries out, depends on whether the animal is terrestrial or aquatic.

Chordates also have a muscular tail.  This tail may only be visible during embryonic development, but it is an essential trait during embryonic development.  Depending on the species, it may shrink if the species no longer depends on it.  The muscular tail extends beyond the digestive tract, and for aquatic chordates especially, this tail is important to propel the animal.

How are chordates different from vertebrates?  The vertebrate definition is a craniate with a backbone.  What is a craniate?  Craniates are all the members of the phylum chordata that have a head.  A head contains a brain at the front end of the dorsal nerve chord, as well as the common sensory organs such as nose, eyes, ears, mouth.  There are actually many invertebrates that have a head, however, a craniate is exclusively a chordate with a head.

To conclude, a vertebrate is a chordate, if, and only if, the chordate is a craniate with a backbone.  A chordate must possess a head and backbone in order for it to be a vertebrate.

(Biology, Lesson 100 essay.)