[Illustration: FIG. 177.—Interior of a portion of the air-bladder of _Neoceratodus_. _av_, Alveolus; _f.r_, the two fibrous ridges. (From Günther.)] The more complicated and much more lung-like air-bladder of _Protopterus_ (Fig. 178)[333] is essentially double, consisting of an anterior unpaired portion, and of two sac-like prolongations which extend backwards the whole length of the coelom, gradually tapering towards the cloaca. Anteriorly, the unpaired portion of the organ is continued into a vestibule or pneumatic duct, which, after passing ventrally on the right side of the oesophagus, opens into the latter by a ventrally-situated, slit-like glottis, immediately behind the last pair of gill-clefts. The margins of the glottis are provided with radially-arranged dilator muscles, and in connexion with its anterior border there is an epiglottis-like fibro-cartilaginous plate.[334] The central cavity of each lung (Figs. 178 and 179) communicates with a series of larger or smaller alveoli in the lung-wall, and each of the latter opens in succession into smaller tubular cavities, and then into still smaller terminal caecal sacculi. Hence, much more than in _Neoceratodus_, the lungs approximate in structure to those of the higher terrestrial {301}Vertebrata. Non-striated muscle cells, pigment cells, and blood capillaries are abundantly present in the connective tissue external to the lining epithelium of the lung-cavities. [Illustration: FIG. 178.—A, the air-bladder of _Protopterus_, viewed from the ventral side. Portions of the ventral walls of the pharynx and bladder have been removed. _gl_; Glottis; _lg_, undivided portion of the lung; _l.l_, left lung; _oes_, oesophagus; _p.a_^{1}, _p.a_^{2}, the left and right pulmonary arteries; _ph_, pharynx; _p.v_, pulmonary vein; _r.l_, right lung; _vb_, vestibule. (From Newton Parker.) B, portion of one lung of _Protopterus_, opened from the dorsal side to show the alveoli. _al_; Alveolus. (From Baldwin Spencer.)] The air-bladder of _Lepidosiren_ closely resembles that of _Protopterus_, and, as in the latter Dipnoid, the glottis seems to be furnished with an epiglottis.[335] In all the Dipnoi the air-bladder is highly vascular, but nevertheless presents no trace of "red bodies" or "red glands." The most striking features in the remarkably polymorphic air-bladder of Teleosts relate to (_a_) its presence or absence; (_b_) differences in shape and relative size; (_c_) the development of caecal outgrowths; (_d_) the subdivision of its cavity by the formation of internal septa; (_e_) the retention or suppression of the ductus pneumaticus, and the occasional development of secondary ducts communicating directly with the exterior; (_f_) the presence of "red glands" or "red bodies"; (_g_) its connexion with the auditory organ; (_h_) its adaptation as an organ for sound-production.