300 FISHES CHAP. 
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. 
The more complicated and 
much more lung-like air-bladder 
of Protopterus (Fig. 178)? is essen- 
tially 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. An- 
teriorly, 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 ventvrally- 
situated, slit-like glottis, imme- 
Fic. 177.—Interior of a portion of the diately behind the last pair of 
air-bladder of Neoceratodus. av, Al- gill-clefts. The margins of the 
veolus ; /.r, the two fibrous ridges. : : 5 s 
(conn Gunther) glottis are provided with radially- 
arranged dilator muscles, and in 
connexion with its anterior border there is an epiglottis-like 
fibro-cartilaginous plate.? 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 eoceratodus, the 
lungs approximate in structure to those of the higher terrestrial 
‘ Newton Parker, Trans, Roy. Irish Acad. xxx. 1892, p. 109 ; Baldwin Spencer, 
op. cit. p. 54. 
* Henle, quoted by Howes, P.Z.S. 1887, p. 501; also Wiedersheim, op. cit. p. 
622 and Fig. 483. 
