THE GENEALOGY OF ANIMALS 81 



— first (after the scaleless, limbless lampreys) the 

 sharks with spiny scales and cartilaginous skeleton, 

 and after these the lung fishes and the bony fishes, 

 with flat, horny scales and skeletons of bone. 

 From the beginning of the Devonian age, when 

 fishes first came into prominence, till the rise of 

 the great reptiles in the Triassic time, fishes were 

 the dominant life of the sea. In the fishes first 

 appeared jaws, a sympathetic nervous system, red 

 blood, backbone, and the characteristic two pairs 

 of limbs of vertebrates. 



The lung fishes (Dipneusta), a small order of 

 strange salamander-like creatures which live in- 

 geniously on the borderland between the liquid 

 and the land, may be looked upon as physiological, 

 if not morphological, links between the fishes and 

 the frogs. They combine the characters of both 

 fishes and frogs, and zoologists have been tempted 

 to make a separate class of them, and place them 

 between the two classes to which they are related. 

 They are like fishes in having scales, fins, per- 

 manent gills, and a fish-like shape and skeleton. 

 They resemble frogs in having lungs, nostrils, an 

 incipiently three-chambered heart, a pulmonary 

 circulation, and frog-like skin glands. There are 

 three genera with several species. One genus 

 (Neoceratodus) is found in two or three small 

 rivers of Queensland, Australia ; another (Protop- 

 terus) lives in the Gambia and other rivers of 

 Africa ; and the third (Lepidosiren) inhabits the 

 swamps of the Amazon region. They all breathe 

 ordinarily by means of gills, like true fishes, but 



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