stages in Hmnan Evolution 



juice," as Mephlstopheles said of blood. But 

 this stage, while exceedingly interesting, is of 

 little practical importance in our study. We 

 must hasten on to the second stage. Here the 

 cells have multiplied greatly, and a certain 

 amount of division of labor has resulted in dif- 

 ferences in their structure. 



We might call it the coelenterate stage, for 

 all the animals which clearly and unmistakably 

 show its type of structure were formerly reck- 

 oned to this subkingdom. Its typical form is 

 seen clearly in our little fresh-water hydra and 

 its allies, less distinctly in the sea anemones, still 

 more modified in the jellyfishes. The old natu- 

 ralist called all these animals zoophytes or plant 

 animals. The chief characteristic of this stage 

 is that the animal is composed of two layers of 

 cells arranged in the form of a double-walled 

 sac. It might be compared to a shopping bag 

 lined with velvet and covered with silk, but with 

 lining and covering glued fast together. Most 

 animals of this type are stationary, but the 

 primitive ancestor probably moved by means of 

 a myriad of microscopic oars shaped like eye- 

 lashes and hence called cilia. 



The opening into the sac is the mouth, the 



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