CHAPTER 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. 
166. 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. 107). 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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