CHAPTER IV 



THE METAZOA, OR THE MULTICELLTJLAR FORMS 

 OP LIFE 



UNLIKE the Unicellular department of 

 life, whose chief characteristic is fixity, 

 that of the Metazoa is development. This 

 development is not alone into a great vari- 

 ety of forms, but much more into a pro- 

 gressive elevation in function, or in the 

 kind of work which is performed. Thus 

 connective tissue cells, whose working is 

 purely mechanical, are inferior to muscle 

 cells, whose chief function is to pull and 

 then to relax, for the relations of muscles 

 to animal heat do not concern us now. 

 But this muscle function of contraction 

 and relaxation is simple compared with 

 the functions of a collection of nerve cells. 

 As function, however, is a gauge of life, 

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