202 THE HAUNTS OF LIFE 
lected by the blood, and got rid of on the skin 
or on the gills, if there are gills. An animal 
like a leech is a good example of cutaneous 
respiration, simply through the skin; a lob- 
worm or a lobster, a mussel or a fish, may 
illustrate respiration by gills. 
But getting on to dry land involved dry 
skins and protected skins, and the diffusing-in 
of oxygen was no longer so easy. Thus we 
find various devices for getting the air into the 
interior of the body and for spreading out 
the blood on internal, not external, surfaces. 
Thus insects evolved air-tubes, carrying fresh 
air to every hole and corner of the body— 
surely part of the secret of their great activity 
—and amphibians evolved lungs, probably 
transformations of the swim-bladder of fishes. 
The lowest animals to show the red-blood- 
pigment (hemoglobin), which we and all back- 
boned animals have, were certain worms called 
Ribbon-Worms or Nemertines, which live for - 
the most parton the seashore. The virtue of this 
hemoglobin is that it captures oxygen very 
readily from outside, and parts with it readily 
to the living tissues, and it is certainly interest- 
ing that some of the Ribbon-Worms have be- 
come terrestrial. There are many backboneless 
animals, such as most of the Arthropods and 
