Describe the structure and location of each of the five key features which define phylum chordata and explain what the function is for each

Phylum Chordata is characterized by five key features: notochord, dorsal hollow nerve cord, pharyngeal slits, endostyle or thyroid gland, and post-anal tail.

1. Notochord: The notochord is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates. It extends the length of the body, providing support and aiding in locomotion. In many chordates, it is replaced by the vertebral column (spine) as the organism develops.

2. Dorsal Hollow Nerve Cord: This is a tube-shaped structure located above the notochord and extends along the length of the body. It develops into the central nervous system: the brain and spinal cord. This feature differentiates chordates from other animal phyla, which have solid, ventrally located nerve cords.

3. Pharyngeal Slits: These are openings that lead from the pharyngeal cavity to the outside. In fish and some amphibians, they develop into gills. In terrestrial vertebrates, they develop into various structures of the head and neck.

4. Endostyle or Thyroid Gland: The endostyle in primitive chordates secretes mucus that traps particles, aiding in feeding. In higher vertebrates, it develops into the thyroid gland, which regulates metabolism.

5. Post-anal Tail: This is an extension of the body that runs past the anal opening. It contains muscle tissue and helps with movement and balance. In many species, it is lost as the individual develops to maturity.

These features are not all present at the same time during the life of most chordates, but at some point during development, they all appear.