Publisher Summary This chapter presents an overview of the internal morphology of gills. Each gill arch skeleton is jointed with the posterior skull dorsally and with the copula ventrally. The septum contains nerves and blood vessels and bears the filaments. Two rows of filaments are generally inserted on each gill arch. This whole forms the so-called holobranchs. The mode of the insertion of filaments on the gill arch depends on the morphology of the septum and varies with the group of fish. In some groups (Chondrostei and the holostean Lepisosteus), certain arches—the hyoidean and the mandibular only—bear a single row of filaments, called “hemibranchs.” In other groups or species, these arches are completely devoid of filaments or absent, such as in Amia (Holostei), which has a mandibular but no hyoidean arch. The presence of lamellae on gill arches corresponds with the distribution of true aortic arches. The pattern of branchial arch organization is relatively constant among teleosts, but differs significantly in lower groups. In Myxinoidei, the variable number of gill slits correspond to a series of pouches connected by narrower tubes with the gut and body surface.