Modification to the pharyngeal apparatus within the vertebrates. Within the vertebrates, the pharyngeal region has undergone extensive modification. The chondrichthyans retain open gill slits, but in the actinopterygian fish, these are covered by a bony operculum, which is derived from the second arch. The tetrapods have undergone the most radical remodelling of the pharyngeal arches as part of their adaptation to terrestrial life. Within this grouping, amphibians possess an opercular flap that fuses at metamorphosis; in amniote embryos, the second arch still expands caudally to cover the posterior arches, but does not retain skeletal elements, and later fuses to the cardiac eminence. The internal gill buds have become modified to form the parathyroid gland.