THE SHAPES AND SIZES OF ANIMALS 151 



smaller one. But the mass of bulk of the animals 

 will be proportionate to the cubes of their linear 

 dimensions i.e., the larger animal will be eight 

 times the bulk of the smaller one. Hence the 

 larger animal, with a bulk or mass eight times that 

 of the smaller one, will have a surface only four 

 times as extensive. This fact, that as animals 

 increase in size their bulk or mass increases at a 

 much faster ratio than their surface, explains how it 

 is that a small animal, such as Pontolimax, may 

 find its body surface sufficient for the interchange 

 of gases that constitutes respiration, while hi a 

 larger animal Doris, of similar shape and constitu- 

 tion, the body surface may be altogether insufficient 

 and special respiratory organs may become neces- 

 sary. Again, in a small animal no part of the 

 body is very far removed from either the surface or 

 from the digestive organs, hence each part is able 

 to effect its respiratory interchange of gases and to 

 obtain its due supply of nutriment directly, without 

 the intermediation of a system of circulatory 

 organs or blood-vessels. In a larger animal how- 

 ever such blood-vessels are absolutely necessary, 

 for without them the more deeply lying parts would 

 be unable to obtain the oxygen which is necessary 

 for their vital activity, or to get rid of the carbonic 

 acid and other poisonous products of that activity ; 

 while the nutrition of the surface and more remote 

 parts of the body could not be properly kept up 

 unless there were direct communication between 

 the digestive organs and these parts. Such con- 

 siderations must suffice to illustrate in what way 



