Chordates and Vertebrates

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

Chordates are a phylum within the kingdom Animalia. They are animals that commonly possess a notochord, a hollow dorsal nerve cord, pharyngeal clefts, and a muscular tail. Some of their traits are only present within their embryonic phase of development.

A notochord is a long and flexible rod-shaped body found between the digestive tract and the dorsal nerve cord and is in all of chordates´ embryos. In some chordates, like certain fish, the notochord remains perpetually to be their area of support for their bodies. The notochord for these animals is made of stiff, fibrous tissues and fluid-filled cells, with which their muscles push against to enable their swimming. In other chordates, the notochord develops in their embryonic stage to further become their spinal discs.

Only chordates have a hollow nerve cord, non-chordate animals have a solid nerve cord, if any at all. Most non-chordate animals have a ventral nerve cord, which means that their stomach resides “on the back” of their bodies. Anyway, in chordates, this nerve cord is created from the outer layer of cells after gastrulation occurs. During this process, a flat plate of cells basically rolls up into a tube. In embryos, the nerve cord will unfold to become the brain and the spinal cord of animals and humans.

A pharynx is a region of the head and neck posterior to the mouth. Chordates have pouches or clefts on the sides of their pharynx, these are known as pharyngeal clefts. For aquatic animals these become into their gills or suspension-feeding organs. In the case of humans and terrestrial animals, these come to be the head and neck structures, like the ears.

Finally, chordates contain a muscular tail that extends beyond their digestive tract. Nevertheless, this structure often shrinks or disappears throughout development. For example, the embryo of humans has this tail that is later reduced to the coccyx or popularly known “tail bone”.

Out of chordates derive craniates. Given this name, we understand that craniates consist of a head. Normally, inside of the head, these organisms carry a brain at the front end of the dorsal nerve cord and they also hold sensory organs in it (eyes, ears, etc.). Craniates own highly complex and coordinated feeding and movement.

Inside the group of craniates are the vertebrates. Vertebrates have an extensive skull and a backbone composed of vertebrae. All animals and humans have homeobox (hox) genes that withhold the specific design for the skeletal, nervous, and muscular structure of each animal. For this reason, every animal species is different, but the same in their group. In other words, all frogs have a frog-like skeletal structure that differs from the dog skeletal structure, thus all frogs are not dogs. Likewise, all vertebrates have hox genes to design for them the brain, skull, and backbone.

Therefore, in conclusion, vertebrates practically differ from chordates in that only vertebrates are completely formed with a skull, a backbone, and a spinal cord, while other chordates are not. In short terms, all vertebrates are craniates and all craniates are chordates, however, not all chordates are craniates and not all craniates are vertebrates.

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