202 THE HAUNTS OF LIFE 



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 lobworm 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 amphib- 

 ians evolved lungs, probably transformations of 

 the swim-bladder of fishes. 



The lowest animals to show the red-blood- 

 pigment (kcemoglobin), 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 interesting 

 that some of the Ribbon-Worms have become 

 terrestrial. There are many backboneless 

 animals, such as most of the Arthropods and 



