2 20 COMPARATIVE ANATOMY 



all the animal forms which have become extinct, it would be 

 impossible to fix the point where the Protozoa ceased and the 

 Metozoa began. The one group must have passed imperceptibly 

 into the other, and it is only because the intermediate forms 

 have become extinct that we are led to draw a hard and fast 

 line between uni-cellular and multi-cellular organisms. And 

 even now the Flagellata, and in a lesser degree the Ciliata, are 

 there to remind us that our definitions are, after all, arbitrary 

 and artificial. 



But, after all, our conception of a Metazoon is not quite 

 simply that of a multi-cellular organism, propagating itself by 

 sexual reproduction, as will appear from the study of the 

 animals described in the following chapters. 



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