112 LIFE IN THE SEA [CH. 



unless it also obtains nitrogenous food, and it must 

 get this from the amino-acids or analogous substances 

 which it finds in solution in the sea. Myriads of 

 marine unicellular animals so obtain their proteid 

 food-stuff from soluble substances and they can do 

 so easily because they are small. If they are very 

 small their surface is large compared with their 

 bulk, and they absorb food-stuff in proportion to 

 their surface. Now let the organism increase greatly 

 in size and its surface must diminish relatively to its 

 volume ; for the surface with increasing size is 

 proportional to the square of the radius, while the 

 volume is proportional to the cube of the radius. As 

 the organism grows in size it must therefore find 

 increasing difficulty in absorbing food-stuff from 

 solution in the sea- water ; for its requirements are 

 proportional to its volume while its powers of ob- 

 taining food are proportional only to its surface. 

 Further, the absorptive surface must become reduced 

 as the animal increases in size, since part of it at 

 least may become thickened to form an integument, 

 so as to afford protection from mechanical injury, 

 and the powers of absorption must therefore be 

 reduced. 



The surface must therefore be increased during 

 the evolution of increasing size, and this is effected 

 by the infolding of a part of the body-wall so as to 

 create an internal cavity, the wall of which may 



