The evolution of water-breathing respiratory faculties in craniotes

Abstract

The major components of the respiratory faculty (gill structure, muscular ventilation, central heart and erythrocyte-containing blood, and pH-sensitive control of breathing) appear to have been present in craniotes from the very beginning. But the details are so different in the most basally radiating group, hagfish, corroborating that they separated very early from the stem line. In the other groups, progressive changes are seen in the structure of the gills, heart, haemoglobin, as well as in the control of breathing. In particular, a major and progressive change in gill structure is seen when comparing sharks to teleosts, with several intermediary forms realized.