information:filtering
Lamprey and chordate synapomorphies

Synapomorphies are certain characteristics that are shared over evolutionary history. Organisms possessing a notochord, dorsal hollow nerve cord, pharyngeal slits, pituitary gland / endostyle, and a post anal tail during the process of their development are considered to be chordates. Lampreys contain these characteristics that define them as chordates. Lamprey anatomy is very different based on what stage of development they are in. The notochord is derived from the mesoderm and is one of the defining characteristics of a chordate. The notochord provides signaling and mechanical cues to help the organism when swimming. The dorsal nerve cord is another characteristic of lampreys that defines them as chordates. During development this part of the ectoderm rolls creating a hollow tube. This is often why it is referred to as the dorsal "hollow" nerve cord. The third chordate feature, which are the pharyngeal slits, are openings found between the pharynx or throat. Pharyngeal slits are filter feeding organs that help the movement of water through the mouth and out of these slits when feeding. During the lamprey's larval stage they feed by filter feeding. Once lampreys reach their adult phase they become parasitic on other fish, and these gill slits become very important in aiding in the respiration of the organism. The final chordate synapomorphy is the post anal tail, which is muscular and extends behind the anus.

Oftentimes adult amphioxus and lamprey larvae are compared by anatomists due to their similarities. Similarities between adult amphioxus and lamprey larvae include a pharynx with pharyngeal slits, a notochord, a dorsal hollow nerve cord and a series of somites that extend anterior to the otic vesicle.