AMPHIBIA 19 



whose appendages are adapted for swimming, 

 although they breathe atmospheric air and are 

 therefore true air-breathers. The cetacea and 

 the sirenia are completely aquatic in so far that 

 if they are driven ashore by storm or accident 

 they are irretrievably lost ; at the same time 

 they can only breathe air. The distinction 

 between air - breathers and water - breathers is 

 therefore more fundamental than that between 

 aquatic and terrestrial animals. The inter- 

 mediaries between these two groups are the 

 amphibious creatures which are by no means 

 confined to the class Batrachia or Amphibia 

 sensu stricto, but include all land crabs, some 

 molluscs, and even certain fishes, both dipnoan 

 and teleostean, as well as some reptiles. 



The essential organs which perform the 

 principal functions of the body might each in 

 turn be made the basis of a physiological classifi- 

 cation which would convey a certain amount of 

 information to those for whom such knowledge 

 possesses any value. More interesting for our 

 present purpose are such groupings as depend 

 upon the relations of animals to their immediate 

 surroundings irrespective of the actual nature of 

 the environment, and it will be seen that the 

 pairs of characters illustrated by the dual sub- 

 divisions which will be discussed below, virtually 

 coincide in their nature with some of the primary 

 differences between animals and plants. 



