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 goup 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- 
