454 ANIMAL RESPIRATION 



cient and once extensive group. They are admittedly amphibious, 

 and have received both their popular and scientific names (Gk. 

 dis, double; pnoe, breath) in acknowledgment of the fact. So 

 much do they differ from other fishes in structure that some zoolo- 

 gists place them in a separate class, and, since in many respects 

 they resemble newts, salamanders, and the like, they may broadly 

 be considered as a sort of half-way stage between Fishes proper 

 and Amphibians. 



The Australian Lung- Fish (Ceratodus] is the least specialized 

 of the three living representatives of its group, and its lung-like 

 swim-bladder is unpaired (but divided by furrows into right and 

 left parts), though it grows out from the under side of the gullet. 

 The lining of this organ is raised into folds, and it returns purified 

 blood to the heart, which we find in consequence to be partly 

 divided into right and left divisions by a partition, so as to keep 

 apart in some degree the two kinds of blood which are poured 

 into it. It is, in fact, the first crude attempt at solving the pro- 

 blem of converting a fish-like type of heart, which receives only 

 impure blood and pumps it to gills for purification, into the kind of 

 heart found in typical air-breathing Vertebrates, which receives both 

 pure and impure blood, that are only disposed of to the best advan- 

 tage when kept separate (see vol. i, pp. 242-244). It is only the 

 two highest classes of backboned animals, i.e. Birds and Mammals, 

 that have attained to a full solution of the problem, and have 

 succeeded in keeping the two sorts of blood completely separate. 

 That the members of these two classes are hot-blooded is one 

 outcome of this feat, and to it they owe, in no small degree, 

 their present position as the dominant inhabitants of the land. 

 For long ages Reptiles were the leading terrestrial forms, but 

 never completely succeeding in converting circulatory organs in- 

 herited from aquatic ancestors into structures thoroughly adapted 

 to air-breathing, ultimately had to give place to Birds and Mam- 

 mals, in which the course of evolution led to more satisfactory 

 results in this and certain other respects. 



Returning from this digression to Ceratodus, it is to be 

 noted that this creature is only found in a remarkably restricted 

 area, being limited, in fact, to two small Queensland rivers, the 

 Burnett and Mary, each of which is practically a chain of deep 

 water-holes, connected by comparatively shallow reaches. Fossil 

 evidence proves that the area of distribution was once very much 



