204 THE HAUNTS OF LIFE 



breathing chambers of land-snails, where the 

 blood is spread out on the roof of a cavity con- 

 taining air, or the true lungs of amphibians and 

 higher vertebrates, should be thought of in con- 

 nection with the fact that land animals tend to 

 become thick-skinned, or to acquire some sort of 

 protection over their skin. An earthworm is 

 still tender-skinned, and it breathes by its 

 skin; a frog is still tender-skinned, and it 

 breathes partly by its skin all through its life, 

 and wholly by its skin in winter. But in the 

 scaly reptiles, in the feathered birds, and in 

 the thick-skinned mammals, usually well-pro- 

 tected besides, all trace of skin-breathing (or 

 cutaneous respiration) has vanished. 



CHANGES IN MOVEMENTS 



Animals in the water have the great advan- 

 tage of universal freedom of movement in any 

 direction. They can go up or down, forward or 

 backward, to right or to left, in any and every 

 plane. But land animals can move only in one 

 plane on the surface of the earth; and this 

 means very great limitations and a great in- 

 crease of risks. It is more than ever necessary 

 that the movements should be quick and pre- 



