AIR-BLADDER 



299 



is still further sacculated by finer branches of the principal 



iibrous bands/ In the Amiidae the bladder ia very larce, 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. 



It may be mentioned that in all 



the preceding Teleostomi the ductus fiq. 175.— Portion of the air 



pneumaticus is remarkably short, the 



connexion between the air-bladder and 



the oesophagus being almost direct by 



means of a larger or smaller oriifice, 



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," 



bodies," or " red glands." 



In the Dipnoi the structural resemblance of the air-bladder 



to a true lung, which to some 

 extent is indicated in Poly- 

 pterus, Amia, and Lepidosteus, 

 becomes still more marked. 



In Neoceratodus'^ the organ 

 is not imlike that of Lepi- 

 dosteus, and takes the form 

 of a spacious unpaired sac, 

 extending from one end of 

 the abdominal cavity to the 

 other. On its inner surface 



bladder, with the ventral 

 wall removed, and the glottis, 

 of Lepidosteus. a.h, Air- 

 bladder ; gl, glottis ; s, bulg- 

 ing of the hinder wall of the 

 vestibnle into the cavity of 

 the air - bladder ; y, cleft 

 leading from the air-bladder 

 into the vestibnle. (From 

 Wiedersheim.) 



red 



" \l!t^i^ 



/b. 



Fig. 176. — Portion of the air-bladder of Lepi- 

 dosteus, opened along the mid-ventral line 

 to show the alveoli, av, Alveolus ; f.b, twO fibrous bands, One of 

 medio-dorsal fibro-muscnlar band. (From ^^lich is dorsal and the Other 

 Wiedersheim.) 



ventral, traverse the whole 

 length of the bladder, and project slightly into its cavity. 



1 Balfour and Newton Parker, Phil. Trans. 173, Part ii. 1883, p. 425. 

 ^ Giinther, Phil. Trans. 161, 1871, p. 511 ; Baldwin Spencer, Zoologische 

 Forsehungsreisen in Australien (Semon), i. Jena 1898, p. 53. 



