CHAPTEE XV 



THE AMPHIBIANS 



IN many respects the amphibians toads, frogs, and sala- 

 manders resemble the fishes, especially the lung-fishes 

 (Dipnoi). The modern amphibians are essentially fishes 

 in their early life, but in developing legs and otherwise 

 changing their bodily form they become adapted for a life 

 on land under conditions differing from those of the fishes. 

 Judging from this class of facts, we may assume that fish- 

 like ancestors, by the development of the lungs, became 

 fitted for a life on land, and that from these the amphib- 

 ians of our times have been derived. 



172. Development. The eggs of the Amphibia are laid 

 during the spring months in fresh-water streams and ponds. 

 They are globular, about as large as shot, and are embedded 

 in a gelatinous envelope (Fig. 108). They are either de- 

 posited singly or in clumps, or festooned in long strings over 

 the water-weeds. During the next few days development 

 proceeds rapidly under favorable conditions, resulting in an 

 elongated body with simple head and tail. In this condition 

 they are hatched as tadpoles. As yet they are blind and 

 mouthless, but lips and horny jaws soon appear, along with 

 highly developed eyes, ears, and nose. External fluffy gills 

 arise on the sides of the head, and slits form in the walls of 

 the throat, between which gills are attached, and over which 

 folds of skin develop, as in the fishes. A fin-fold like that 

 of the lancelet or lamprey appears on the tail. The brain 

 and spinal cord, extending along the line of the back, are 

 supported by a gristly notochord, and complete and com- 

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