34 



Anipnir)ians — Tne Frog^ 



An Example or tlie Vertebrates 



Salamanders, frogs and toads, and the little-known wormlike caecilians live 

 partly in water and partly on land; hence the name. Class Amphibia, from 

 amphibios meaning double living. All of them spend part of their life span in 

 the water. A very rare and specialized few stay in it all their lives. From the 

 fossils that picture their early history, it appears that amphibians originated 

 from fishes, that some of those pioneers of ancient times lost their scaliness 

 and became the ancestors of modern frogs while others kept their scales and 

 gave rise to reptiles. Amphibians are the oldest four-footed backboned animals, 

 once dominant in the swamps of the early Mesozoic Period 200 or more 

 million years ago. In times long before paddles were transformed into legs, the 

 air-breathing lobe-fin fishes must have been stranded in muddy water full of 

 gas from decaying vegetation. A few were mired in the clay and became fossils. 

 Others wriggled into fresh pools and shady places. After millions of years of 

 natural selection their descendants managed to walk on their weak legs shifting 

 their bodies from side to side, as salamanders still do (Fig. 34.2). 



Characteristics. Amphibians are vertebrates with moist glandular skins and 

 no external scales. Except for the limbless caecilians, they have two pairs of 

 limbs used in walking or swimming (Fig. 34.1). The two nostrils connected 

 with the mouth cavity have valves to shut out the water. The heart has two 

 auricles and one ventricle. Respiration is by gills, lungs, skin, the lining of the 

 mouth, or combinations of these. There are gills at some phase of the life span, 

 e.g., in the tadpoles of frogs. The eggs are fertilized externally in frogs and 

 toads, internally in salamanders. No membranes are formed around the embryo 

 (Chap. 19). 



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