Photoreception in hagfishes: Insights into the evolution of vision

Introduction Hagshes are the only surviving members of the group of jawless, marine chordates or craniates that diverged from the ancestors of lampreys more than 500 million years ago (mya). Previous studies have revealed that many characteristics about the anatomy and physiology of hagshes appear “simple” or “primitive” and even resemble nonvertebrate chordates such as amphioxus and sea squirts (Forey and Janvier, 1993; Lacalli, 2004; Janvier, 2007). These features include differences in the vertebral column, the ability to control water content by osmoregulation, and the presence of lymphocytes.