Chordate Evolution

My assignment was to "[c]reate a project that shows the evolutionary developments that helped to lead to true vertebrates in the following animals. Begin with the most primitive, and work your way up to the most advanced.

Amphioxus

Hagfish

Hemichordates

Lampreys

Tunicates"

Hemichordates came first, and they had two of the distinguishing features of chordates: gill slits and a dorsal nerve cord. (They also had a ventral nerve cord.)

Next came the Tunicates, or at least their larvae. They have the dorsal nerve cord and pharyngeal gill slits, like their predecessors, but they also have something new: the notochord. Thus, they have three of the four distinguishing chordate characteristics. However, most of these features are lost when they become adults, leaving only the gill slits behind.

Amphioxus was the first organism to have all four chordate features as an adult. The fourth chordate feature is a post-anal tail.

The hagfish was the first agnathan, and just barely scrapes across the boundary to vertebrates. Though more advanced than amphioxus, its spine (derived from the notochord) was a single cartilaginous rod, and it has no jaw.

The next step was the Lamprey. It had all the chordate features, plus fins, and individual vetebrae.( It still no jaw, though.)