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 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.
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.a1, p.a2, 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.
(a) The air-bladder is by no means universally present in Teleosts. It is absent in several entire families,[[336]] such as, for example, the Flat Fishes or Pleuronectidae, the Scopelidae, and the "Lump-suckers" (Cyclopteridae). In a few families, as in the Mackerels (Scombridae), the "Blennies" (Blenniidae) and the Polynemidae, the organ is present in most genera, but absent in a few, or even present or absent in different species of the same genus. Thus, of the three British species of Mackerel, viz. the Spanish Mackerel (Scomber colias), S. pneumatophorus, and the common Mackerel (S. scombrus), an air-bladder is present in the first two, but absent in the third.[[337]]
(b) As might be anticipated, the shape of the air-bladder is extremely different in various Teleosts, and usually conforms to the shape of the body, while differences in relative size are of frequent occurrence, even in closely related species. Sometimes the organ is more or less tubular, fusiform, ovoid, or heart-shaped; occasionally it is shaped like a "dumb-bell," consisting of two lateral sacs connected by a median tubular portion, as in the Siluroids Clarias and Callichthys; or it may be horse-shoe-shaped, as in the Silurid Ailia.[[338]] Not unfrequently a transverse constriction divides the air-bladder into two intercommunicating sacs, as in most of the Carp family (Cyprinidae), or three such sacs may be formed by two constrictions (e.g. Ophidium). In the "Electric Eels" (Gymnotidae) there are two sacs, connected by a slender canal, from which the ductus pneumaticus takes its origin.[[339]]
Fig. 179.—Showing the structure of one of the larger alveoli of the air-bladder of Protopterus. 1, Central cavity of the lung; 2, alveolus; 3, tubular cavities communicating with 4, the small terminal sacculi. (From Baldwin Spencer.)