Amphioxus

Lancelet, also known as amphioxus, is said to be the most primitive example of a vertebrate.

Lancelets are the modern representatives of the subphylum Cephalochordata. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially as food for humans and domesticated animals. Cephalochordates are small, eel-like unprepossessing animals. The reason lancelets hold such importance in the study of zoology is because of their remarkable morphology, providing an understanding of the morphology and evolution of chordates, including vertebrates.

Below is an external and internal view of the amphioxus cephalochordate. Cephalochordates have all the typical chordate features. The dorsal nerve cord is supported by a muscularized rod called the notocord. the pharynx is perforated by over 100 pharyngal slits, or "gill slits," which are used to strain food particles out of the water. The musculature of the body is divided up into V-shaped blocks called myomeres. There is also a post-anal tail. All of the above named features are shared with vertebrates. Arguably, there are features that vertebrates have that cephalochordates lack: the brain and sense organss are poorly developed in the amphioxus, and there is true vertebrae in the organism as well.

The fossil record of cephalochordates is extremely sparse because they have no hard parts. However, fossil cephalochordates have been found in very old rocks, predating the origin of the vertebrates. The fossils show that the chordate lineage appeared very early in the known history of the animal kingdom, and they strengthen the case for an origin of true vertebrates from a cephalochordate-like ancestor. 

Check out this video on Chordate Evolution!