CLASSIFICATION 35 



to make for them a special group, the Mesozoa, 

 or Midway animals, between the Protozoa and all 

 the rest of the animal kingdom. It is, however, 

 possible to group them under the head of Diplo- 

 blastic animals; but nothing more different from 

 the Coelenterata could well be imagined, and some 

 regard them as a degraded form of worm. 



The animals which are higher in structure than 

 the Protozoa, viz. our divisions 2 to 10, are often 

 grouped under the name 

 Metazoa. The Metazoa 

 thus include Grades II., 

 III., and IV. 



The meaning of the 

 division of the animal 

 kingdom into grades 

 will be more apparent 

 if we give an example 



of each. cv, contractile vacuole ; ps, 



fiRAnp T Thf Onf pseudopodia ; highly magni- 



UrRADE 1. 2/ieyne- fied This represents Grade 



Celled Animals. Amce- I. of animal existence. 



ba, the Mobile animal, 



is the typical example of these. It consists of a 

 single microscopic cell. In this cell is seen a 

 dark irregular speck, the nucleus, which is an 

 essential character of cells, whether they are in- 

 dependent or form part of the body of a larger 

 animal. There is often visible also a clear 

 rounded space, called the "contractile vacuole," 

 which squeezes out fluid, disappears, and reap- 

 pears again, serving the purpose of excretion. 

 The cell-substance, called protoplasm, is prac- 

 tically identical in this and in cells of all other 

 kinds. It is jelly-like, and capable of a slow 

 movement, which may be watched under the mi- 

 croscope. It suggests the flowing of treacle or 



