Wednesday, July 31, 2013

What are Chordates Anyway?

Our skinless friend above might appear to be just another fish of sorts, but sadly you'd be mistaken if that's what you assumed. The only thing this creature really has in common with fish actually is that at some point in it's life cycle it has four distinguishing features that allow us to classify it as a chordate. Lancelets, tunicates, fish and even we humans are all under the prestigious umbrella of chordates.

So what makes us all so special? Well as I mentioned before, there are no less than four features that chordates share at some point. I'll talk about each in turn but in short they possess a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail. A key point to remember is that chordates possess these "at some point", so while we humans are chordates, don't expect to see anyone with a tail walking about. Although there's always the odd case, most of us only possessed this vestigial structure while still an embryo in the womb.

Members of the subphylum Cephalochordata, lancelets are great examples of chordates, possessing all the traits in the larval stage and maintaining most of them into adulthood. Other chordates include tunicates (Urochordata), hagfish (Myxini) and all vertebrate animals. The four trait's that make them chordates are as follows:

1. Notochord: A flexible rod running the length of the body, this skeletal support structure is located along the back. In vertebrates it becomes encased in the backbone structures know as vertebrae.

2. Hollow Nerve Cord: The nerve cord lies behind the notochord and is in fact hollow. Nonchordates tend to have solid nerve cords. For chordates this structure becomes the central nervous system, spinal cord and brain.

3. Pharyngeal Slits: In chordate embryos a series of grooves and pouches form along the pharynx, the region behind the mouth. In many chordates these form slits that allow water to enter the mouth and exit the body without entering digestive system. These structures are used for feeding in some and for breathing in other chordates such as fish.

4. Post-Anal Tail: Like the name says chordates have a tail that extends past the anus. In humans this structure is only present in the embryo and disappears before birth.

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