The lungs of the Urodela are always simple, extremely thin-walled bags. They are highly developed in the Anura, the walls being modified into numerous air-cells, whereby the respiratory surface is considerably increased. The lungs are filled with air by the pumping motion of the throat while the mouth is closed, the nostrils being provided with muscular valves. A muscular apparatus assists the filling of the lungs in the Anura.[23] [Illustration: FIG. 6.–Internal view of the mouth of A, _Rana esculenta_, B, _Bufo calamita_ (cf. Fig. 52, p. 269). _Ch_, Choana, or inner nasal opening; _E_, opening of the Eustachian tube; _S_, slit leading into the vocal sac; _T_, tongue; _Vo_, patches of teeth on the vomers.] Most, if not all, Anura and some Urodela have a VOICE produced by the larynx, which, especially in the Anura, is provided with a complicated cartilaginous and muscular apparatus and with vocal cords. The voice of the Urodela is at the best a feeble squeak. The females of the Anura are either mute or they produce a mere grunt, but that of many males is very loud, and, moreover, in many species it is intensified by _vocal sacs_ which act as resonators. These sacs are diverticula of the lining of the mouth-cavity, and bulge out the outer skin and the muscles, chiefly the mylo-hyoid, of the throat. The nostrils and the mouth are firmly closed during the croaking. "The sacs are called internal when they are covered by the unmodified gular integument, however much this may be distended; external when their membrane projects through slits at {48}the sides of the throat, as in _Rana esculenta_ (Fig. 52, p. 269), or when the skin is thinned and converted into a bladder-like pouch, as in _Hyla arborea_."[24] These sacs exhibit many modifications. They may be unpaired and median, and open by two slits into the mouth, on either side below the tongue; in _Bufo_ one of the slits or openings, either the right or the left, is obliterated. They may be paired and symmetrical, and open one on each side of the head, below and near the posterior angle of the jaws. These modifications differ in closely allied species. They reach their greatest complication in _Rhinoderma_ and in some of the Cystignathidae by extending far back beneath the skin into the wide lymphatic spaces. In _Rhinoderma_ they are put to the unique use of nurseries for the young (see p. 228). _Leptodactylus typhonius_ has a very distinct pair of outer vocal sacs and a well-marked unpaired sac which extends into the belly and communicates with each outer sac. Several species of _Paludicola_, e.g. _P. fuscomaculata_ and _P. signifera_, have a similar arrangement, in addition to an unpaired gular sac which can be inflated independently of the rest (see Fig. 45, p. 220). URINO-GENITAL ORGANS The kidneys and the male generative glands are still intimately connected with each other. The general plan is as follows:–