110 How Animals Changed 



in mud in shallow water. It was found to have a groove on its back 

 in which it carried down a bubble from the surface. 



There is abundant evidence from the past and present which 

 indicates that air-breathing originated on beaches and in stagnant 

 water long before life developed on land (Pearse, 1930, 1932b) . 

 Hall (1924) believes that primitive cartilaginous fishes (Chond- 

 rostei) used vascular enteric membranes for breathing from air that 

 they had gulped in. One line of development (Holostei, Teleostei) 

 from these emphasized the swim-bladder as a respiratory-hydrostatic 

 organ, and another line (Crossopterygii, Dipnoi, Amphibia) de- 

 veloped a lung. Hall also points out that toads have improved their 

 lung as an organ for use on land over that of aquatic salientians 

 by enclosing the alveolar sacs more completely, so that they better 

 conserve water. Reptiles have further isolated lobules by the devel- 

 opment of branching bronchi. "The respiratory organs of terrestrial 

 vertebrates are lungs. These arose in phylogeny long before the land 

 was invaded" (Noble, 1931). "Aerial respiration was apparently 

 first achieved by ganoid forms higher than the elasmobranchs, but 

 ancestral to the Crossopterygii and Dipnoi. ... A lung in the form 

 of an air bladder opening off the ventral surface of the esophagus 

 was present in the early ganoids before the line leading to the early 

 Dipnoi, the Crossopterygii, and the Amphibia was separated from 

 the parent stem" (Smith, 1931). A primitive type of lung persists 

 today in the Dipnoi. "The breathing of atmospheric air had al- 

 ready been acquired by several groups of fishes of the ancient coal 

 swamps, as it has by several unrelated modern fishes. If we may 

 judge from modern conditions, an oxygen-secreting pouch long 

 served to tide the fish over periods of drought, and possibly the 

 stout fan-shaped paddles assisted them in wiggling from one pool 

 to another. When breathing by the air-sac finally superseded breath- 

 ing by gills, in the adult stage, it is not surprising that the opercular 

 bones, which play an important part in branchial respiration, should 

 have failed to ossify, leaving only a dermal flap. The region of the 



