n] BIOLOGICAL FOUNDATIONS 63 



place, metabolism can be maintained in spite of 

 increased size, since conducting channels for the food 

 and waste-products can be constructed. 



This would be all but impossible within the limits 

 of an enlarged single cell, owing to the semi-fluid 

 nature of its protoplasm; but now since each cell 

 has a firm outer wall, by joining cell to cell tubes 

 can be made through which the food and the waste- 

 products can quickly pass from end to end of the 

 organism instead of having to work gradually through 

 by diffusion. At first, as in flatworms, most of the 

 various systems of the body the digestive, the 

 genital, and the excretory are themselves profusely 

 branched, but later the whole business of distribution 

 and collection is taken over by the circulatory system : 

 this alone is ramified and the others can pursue their 

 more proper avocations in peace 1 . 



The nervous system is another which can be 

 much more easily perfected in an individual of the 

 second grade. To perform complicated actions which 



1 There is in many protozoa a form of circulation known as 

 cyclosis, in which the whole inner part of the cell is constantly 

 revolving. This certainly performs the same general functions for 

 the organism as does a blood-system, but to have the whole of one's 

 inside always in motion would render difficult the development of 

 other systems ; thus a huge single cell with cyclosis would have over- 

 come the difficulty of metabolism, but would be at a disadvantage in 

 other ways when compared with a multicellular organism of the 

 same size. 



