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 part on the seashore. The virtue of this 

 haemoglobin 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 



