204 THE HAUNTS OF LIFE 



breathing chambers of land-snails, where the 

 blood is spread out on the roof of a cavity 

 containing air, or the true lungs of amphibians 

 and higher vertebrates, should be thought of in 

 connection 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-protected besides, all 

 trace of skin-breathing (or cutaneous respira- 

 tion) 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 precise ; 



