384 FISHES 



present in Polypterus, although the inner surface of the bladders 

 is not sacculated as in Lepidosteus. 



In the living lung-fishes the respiratory function of the air- 

 bladder is of course quite obvious, and in this respect they are 

 intermediate between fishes and Amphibia. In Ceratodus, the 

 Australian lung-fish (Plate XXXII., A), only one sac is present 

 which lies above the intestines and extends from one end to the 

 other of the body-cavity. The sac is large and wide, and along 

 the median line dorsally and ventrally are two fibrous bands 

 projecting into the cavity. Transverse bands passing between 

 these on either side form the borders of two series of lateral 

 chambers, which are again divided by projections of the wall 

 into smaller alveoli, so that the respiratory surface is much in- 

 creased as in a lung, and the wall is richly supplied with blood- 

 vessels. The pneumatic duct, corresponding to a trachea or 

 windpipe arises from the anterior end of the bladder on the right 

 side, passes to the right of the gullet, and opens into the latter 

 by a slit on the ventral side a little to the right of the middle line. 

 We may conclude therefore that the whole bladder in Ceratodus 

 is only the bladder of the right side, that of the left, which is 

 smaller in Polypterus, having entirely disappeared. 



In the African lung-fish Protopterus, and the South Ameri- 

 can Lepidosiren, the air-bladder although single anteriorly is 

 completely divided posteriorly for the greater part of its 

 length, but even in these the gullet does not pass between the 

 right and left sacs, but the air-duct passes to the right of the 

 oesophagus and opens into the ventral side of the latter. The 

 structure, however, is so symmetrical that it is scarcely possible 

 to regard it as formed by the division of a single sac like that 

 of Ceratodus belonging to the right side : it appears rather as a 

 median divided outgrowth of the gullet which has passed to the 

 side of the oesophagus and become dorsal in position. 



We have seen that the lung-fishes instead of being recently 

 evolved modifications of the fish tribe are shown by their fossil 

 remains to be exceedingly ancient. It might be suggested that 

 although they themselves are ancient they have only recently 

 evolved lungs, or recently begun to use the air-bladder for 

 respiratory purposes. The earliest fossil forms, however, possess 

 many of the special characters which distinguish the living 

 lung-fishes from other fishes, such as the lobate paired fins, 



