In the Lepidosteidae the single air-bladder extends the whole length of the abdominal cavity, and, as in _Polypterus_, communicates with the exterior through a larynx-like vestibule provided with a glottis,[330] which, however, opens dorsally into the oesophagus (Fig. 175). A strong fibrous band runs along the median line of the inner surface of its dorsal wall, from which extends ventrally on each side a series of transverse fibro-muscular ridges, forming the boundaries of a double row of regularly arranged alveoli (Fig. 176). The bottom of each alveolus {299}is still further sacculated by finer branches of the principal fibrous bands.[331] In the Amiidae the bladder is very large, and, except that a short median cleft divides it in front into two short caeca, it is unpaired. Internally, its walls are much sacculated, but the alveoli are smaller and arranged less regularly than in _Lepidosteus_. The aperture of communication with the oesophagus is dorsally situated. [Illustration: FIG. 175.—Portion of the air-bladder, with the ventral wall removed, and the glottis, of _Lepidosteus_. _a.b_, Air-bladder; _gl_, glottis; _s_, bulging of the hinder wall of the vestibule into the cavity of the air-bladder; _v_, cleft leading from the air-bladder into the vestibule. (From Wiedersheim.)] It may be mentioned that in all the preceding Teleostomi the ductus pneumaticus is remarkably short, the connexion between the air-bladder and the oesophagus being almost direct by means of a larger or smaller orifice, which, except in _Acipenser_, is more anteriorly placed than in most other Teleostomi; and further that, unlike many Teleosts, there are no special "retia mirabilia," "red bodies," or "red glands." [Illustration: FIG. 176.—Portion of the air-bladder of _Lepidosteus_, opened along the mid-ventral line to show the alveoli. _av_, Alveolus; _f.b_, medio-dorsal fibro-muscular band. (From Wiedersheim.)] In the Dipnoi the structural resemblance of the air-bladder to a true lung, which to some extent is indicated in _Polypterus_, _Amia_, and _Lepidosteus_, becomes still more marked. In _Neoceratodus_[332] the organ is not unlike that of _Lepidosteus_, and takes the form of a spacious unpaired sac, extending from one end of the abdominal cavity to the other. On its inner surface two fibrous bands, one of which is dorsal and the other ventral, traverse the whole length of the bladder, and project slightly into its cavity. {300}Between these median ridges extend a number of transverse septa, forming the boundaries of a series of pairs of bilaterally symmetrical oval alveoli, the walls of which are still further sacculated by a network of finer ridges (Fig. 177). The short ductus pneumaticus seems to be an anterior continuation of the right half of the bladder, and opens into the oesophagus by a small glottis, situated on the ventral side, a little to the right of the median line.