26 

 The Transition to Land 



Amphibians and Reptiles 



As we learned in the last chapter the first three classes of vertebrates 

 are water animals. The transition to life on the land, such as is charac- 

 teristic of most of the advanced vertebrates, is not accomplished com- 

 pletely in any one animal, but through a series of adaptations extending 

 over two classes. These two classes of vertebrates will be studied in this 

 chapter. 



Class— Amphibia 



This class includes the frogs, toads, and a few primitive, tropical, 

 worm-like forms. The class name comes from Greek words meaning 

 both forms of life, and animals of this group are given such a name be- 

 cause they really lead a double life ; they are both water and land animals. 

 All the vertebrates studied up to this time have been water animals and 

 the amphibians start their life as if they were going to keep up the tradi- 

 tion. They hatch out of the egg as fish-like animals with three fully 

 developed and functional gills which are used for respiration. After a 

 time, however, certain changes occur which transform them into animals 

 adapted to live on the land. The gills gradually disappear and lungs ap- 

 pear, small limb buds appear which grow into legs, and a neck appears 

 to give the head movement independent of the trunk. These animals 

 are now able to crawl out on the land and take their place as terrestrial 

 vertebrates. Some of them have made quite a success of their life on 

 land, while others seem to have found this life too rigorous and have 

 crawled back in the water to spend the greater part of their time there, 

 even though they must come to the surface to breathe. A few of these 

 do not give up their larval gills and use both gill and lung respiration. 



In making the transition to land, however, the amphibians have re- 

 tained some of the methods of living characteristic of aquatic animals. 

 Although some of them are capable of living on the land far from the 

 water, when the time comes for reproduction they must return to their 

 ancestral home in the water to accomplish this vital function. There 



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