THE EIGHT PRIMARY GROUPS OF ANIMALS. 85 



duction of these animals, they are, to a certain extent, working 

 in the dark. 



In conclusion, I may sum up the results of this lecture by 

 stating that, in the present state of our knowledge, the whole 

 Animal Kingdom is divisible into eight primary categories or 

 groups, no two of which are susceptible, in the present state of 

 knowledge, of being defined by characters which shall be at 

 once common and diagnostic. 



These groups are the — 



Vertebrata. 

 mollusca. annulosa. 



molluscoida. annuloida. 



c(elenterata. infusoria. 



Protozoa. 



I leave aside altogether the question of the equivalency of 

 these groups ; and, as I have already stated, I entertain some 

 doubts regarding the permanency of one — the Infusoria — as a 

 distinct primary division. Nor, in view of the many analogies 

 between the Mollusca and the Molluscoida, the Annulosa and 

 the Annuloida, do I think it very improbable that, hereafter, 

 some common and distinctive characters may possibly be dis- 

 covered which shall unite these pairs respectively. But the 

 discoveries which shall effect this simplification have not yet 

 been made, and our classification should express not anticipa- 

 tions, but facts. 



I have not thought it necessary or expedient, thus far, to 

 enter into any criticism of the views of other naturalists, or to 

 point out in what respect I have departed from my own earlier 

 opinions. But Cuvier's system of classification has taken such 

 deep root, and is so widely used, that I feel bound, in conclu- 

 sion, to point out how for the present attempt to express in a 

 condensed form the general results of comparative anatomy 

 departs from that embodied in the opening pages of the " Kegne 

 Animal." 



The departure is very nearly in the ratio of the progress of 

 knowledge since Cuvier's time. The limits of the highest 



