The head shield had a series of grooves over the whole surface, forming an extensive lateral line organ.

The mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.

A fleshy appendage emerged laterally on each side, behind the head shield, functioning as pectoral fins.

[4] In the 1920s, the biologists Johan Kiær and Erik Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.

Many biologists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and the affinities of the lampreys are also contested.

[6] Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class whose sole living representatives are the lampreys.