CHAPTER XV. 



THE CLASSIFICATION OF ANIMALS. 



Between Amxba or Vortkella, on the one hand, and Hydra 

 on the other, is a great gap. The two unicellular organisms 

 are representatives of the Protozoa, while Hydra and all the 

 other animals dealt with in this book are Metazoa. These 

 two fundamental divisions of the animal kingdom are some- 

 times — and inaccurately — defined as being respectively uni- 

 cellular and multicellular animals. This definition would, it 

 is true, suffice, if we had only the animals described in the 

 present book to deal with. But there are organisms exceed- 

 ingly like Voi-iicella, which yet form colonies branching from 

 common stalks ; but the individuals forming these colonies are 

 independent of each other, and each feeds itself and propagates 

 its kind on its own account. So that mere multicellularity is 

 not the essential difference between Protozoa and Metazoa. 

 The hydra is not only multicellular, but the cells are specialised 

 in various directions. There are digestive cells, muscular 

 cells, and so forth. Even at this point we have not reached 

 the real difference of metazoon from protozoon ; for, in Volvox, 

 a colonial protozoon, there are special cells set apart for re- 

 production, while something of the same kind occurs in the 

 colonial form Proterospongia, where there are amoeboid and 

 more highly specialised cells imbedded in the same mass of 

 supporting jelly. 



The essential difference between the protozoa and the 

 metazoa are two. 



I. The Metazoa consist, either temporarily or permanently, 

 of a two-layered sac surrounding a central cavity, which opens 

 on to the exterior at one end, the cells of the inner layer 



