The general statement that the Amphibia breathe by lungs, and, at least during some stage of their life, also by gills, requires various restrictions. As a rule the majority of Amphibia first develop gills, later on also lungs, whereupon, during the metamorphosis, the gills are gradually suppressed, so that the perfect animal breathes by lungs only (see p. 61). But a number of Urodela retain their gills throughout life, although the lungs are also functional. These are the Perennibranchiata, not a natural group, but a heterogenous assembly, Proteidae and Sirenidae. Some species of _Amblystoma_ remain individually Perennibranchiate (cf. Axolotl, p. 112). On the other hand, in some Anura the gills are almost or entirely suppressed, or restricted to the embryonic period only. Lastly, a considerable number of Salamandridae have lost their lungs; they breathe by gills until their metamorphosis, but have in the adult state to resort to respiration by the skin (cf. p. 46). The general plan of the development of the branchial respiratory apparatus is as follows:–The six visceral arches, {41}namely, the mandibular, the hyoidean, and the four branchial arches, correspond, long before they are cartilaginous, with four main arterial arches of the truncus arteriosus. The first, the arteria hyo-mandibularis, belongs to the hyoidean and mandibular segments, the second to the first branchial, the third to the second branchial, while the fourth soon splits in two for the third and fourth or last branchial arch. On the dorsal side these branchial arterial arches combine to form the radix of the dorsal aorta. These arches, especially the three branchials, appear in newts, less clearly in frogs, as transverse ridges on the sides of the future neck. Between the arches the pharynx gradually bulges out in the shape of five lateral gill-pouches; the first between the mandibular and the hyoidean arch, the second between the hyoidean and the first branchial arch, etc. These pouches soon break through to the outside and become gill-clefts, except the first pouch in Urodela. _Before_ the breaking through of the clefts there appears upon the outside of the middle of the rim of each arch a little knob, which soon ramifies and forms an external gill. The knob owes its origin to the development of a blood-vessel which buds from the arterial arch, ramifies and breaks up into capillaries, and returns a little further dorsalwards into the arch. A secondary loop to the outside of the primary arterial arch is thus formed; and whilst this outer loop sprouts out further, driving before it the likewise proliferating skin, and thus producing the gill, the middle portion of the primary arch remains in the Urodela as a short cut, but in the Anura it partly obliterates, and henceforth acts as the internal _efferent_ vessel of the gill. When, during metamorphosis, the gills disappear, their intrinsic _afferent_ and _efferent_ vessels vanish