What Is a Fish? 15 



own hammer and awl. For the fish has many cells, and these 

 cells are grouped together into what we may call organ- 

 systems, and each of these systems takes care of one or more 

 of the principal activities which are necessary to the fish's 

 life. Like the modern factory, the fish has gone in for di- 

 vision of labor. Unlike the modern factory, he has not put 

 his simple predecessor out of business, for protozoans still 

 prosper mightily upon this earth. Although most of them 

 are too small for us to see, their number is legion, and some 

 have developed their single cell into most beautiful and 

 even elaborate structures. 



