CHAPTER VI. 



THE FIRST AIR-BREATHERS. 



WERE our experience limited to the animals whose 

 remains are found in the earlier Palceozoic rocks, we 

 might be unable to conceive the possibility of an animal capable 

 of living and breathing in the thin and apparently uncongenial 

 medium of air. More especially would this appear doubtful if 

 our experience of the atmosphere presented it to us as loaded 

 with carbonic acid, and less rich in vital air than it is at present. 

 Even the mechanical difficulties of the case might strike us as 

 considerable, in our ignorance of the capabilities of limbs. 

 Still, as time wore on, we should find this problem worked out 

 along three distinct lines of advancement— those of the Mol- 

 lusk, the Arthropod, and the Vertebrate, and in each of these 

 with different machinery, related to the previous locomotive 

 and water-breathing apparatus of the type. 



Respiration under water depends, not on the water itself 

 but on the small percentage of free oxygen which it contains' 

 and this is utilised for the aeration of the blood of animals, by 

 that wonderful and often extremely beautiful apparatus of deli- 

 cate fibres or laminae penetrated with blood-vessels, which we 

 call a gill. Except those lowest creatures which aerate their 

 blood merely at the general surface of the body, all animals 

 capable of respiration in water are provided with gills in some 



