CHAPTER I. 



THE CLASSIFICATION OP ANIMALS. 



The classification of Cuvier, ia the year 1812, which 

 divided the animal kingdom into the four great groups 

 of Vertebrata, Mollusca, Articulata, and Radiata, is 

 the foundation of that now received. Previous to that^ 

 the best classification was that of Linnteus : it onlj' 

 provided for two groups of invertebrates, the Insecta, 

 including the animals still called Insecta, and the 

 Worms (Vermes), under which heading were squeezed 

 in all the rest of the animal kingdom, including not 

 only what we call worms, but also the moUusca, or 

 shell-fish, the zoophytes, or radiate animals, and the 

 infusoria, or microscopic animals, so called because 

 they abound in infusions of animal or vegetable 

 matter. The classification of Cuvier was a great ad- 

 vance in its recognition of the fundamental importance 

 of the difference between radial symmetry and bila- 

 teral metameric symmetry. " The most essential of 

 the modifications which it has become necessary to 

 make in Cuvier's system relate chiefly to the increase 

 of the number of types. The Infusoria were some 

 time ago removed from the Radiata, and as Protozoa 

 arranged by the side of the other four groups. Lately 



