300 



FISHES 



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) 1 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 ventrally- 

 situated, slit -like glottis, imme- 



diatel 7 behind the last P air f 



air-bladder of Neoceratodus. av,Al- gill- clef ts. The margins of the 

 veolus ; f.r, the two fibrous ridges. -, ... -j i -;i T n 



(From Giinther.) glottis are provided with radially- 



arranged dilator muscles, and in 



connexion with its anterior border there is an epiglottis-like 

 fibro-cartilaginous plate. 2 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 



1 Newton Parker, Trans. Roy. Irish Acad. xxx. 1892, p. 109 ; Baldwin Spencer, 

 op. cit. p. 54. 



2 Henle, quoted by Howes, P.Z.S. 1887, p. 501 ; also Wiedersheim, op. cit. p. 

 622 and Fig. 483. 



FiG.177.-Interiorofaportionofthe 



