 In fish, the other posterior arches contribute to the branchial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus.The genetic and developmental basis of pharyngeal arch development is well characterized.It has been shown that Hox genes and other developmental genes such as "DLX" are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches.