CHAPTER XXIII 



THE FROG : — A STUDY OF THE ORIGIN OF LAND 



LIFE 



It is certain that the primitive animals lived in water because 

 all the lowest forms that we know to-day are aquatic and dry 

 up at once if left in the air. Only some of the higher and 

 more specialized forms have been able to live on dry land. 

 Such are the insects, spiders, certain parasitic forms, and the 

 higher vertebrates. There are a few scattered, terrestrial rep- 

 resentatives of other groups, such as the earthworms, certain 

 Crustacea (the wood-hce), and land snails. Animals that are 

 fitted to live in water find that they are not well adapted to 

 the land because of the great differences in the two habitats. 

 In the first place, water animals usually have only a thin skin. 

 This would not protect the body from loss of water in the air. 

 Consequently, only the thicker skinned aquatic animals can 

 become terrestrial. Second, such thick-skinned aciuatic 

 animals must have gills for breathing. These thin-walled 

 structures cannot exist in dry air. Breathing organs of land 

 animals must be placed deep in the body so that the air is 

 moistened by the body before it reaches them. Again the 

 weight of the body is supported in the water by its buoyancy, 

 so that the appendages are placed laterally and function as 

 oars. But on the land the body must be supported by the 

 appendages on the solid substratum, consequently the appen- 

 dages are placed below the body. Finally, since all those 



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