MODERN SYSTEMS. 289 



Khizopoda produce any eggs, I shall have more to say 

 presently. As to the arrangement of the leading groups, 

 Vertebrata, Articulata, Cephalopoda, Mollusca, Venues, 

 Eadiata, and Protozoa in Vogt's system, it must be appa- 

 rent to every zoologist conversant with the natural affini- 

 ties of animals, that a classification which interposes the 

 whole series of Mollusks between the types of Articulata 

 and Worms cannot be correct. A classification based, 

 like this, solely upon the changes which the yolk under- 

 goes, is not likely to be the natural expression of the 

 manifold relations existing between all animals. Indeed, 

 no system can be true to nature which is based upon the 

 consideration of a single part or a single organ. 



After these general remarks, I have only to show more 

 in detail why I believe that there are only four great 

 fundamental groups in the animal kingdom, neither more 

 nor less. 



With reference to Protozoa, first, it must be acknow- 

 ledged, that, notwithstanding the extensive investigation 

 of modern writers upon Infusoria and Ehizopoda, the true 

 nature of these beings is still very little known. The 

 Ehizopoda have been wandering from one end of the 

 series of Invertebrata to the other, without finding a 

 place generally acknowledged as expressing their true 

 affinities. The attempt to separate them from all the 

 classes with which they have been so long associated, and 

 to place them with the Infusoria in one distinct branch, 

 appears to me as mistaken as any of the former arrange- 

 ments ; for I do not even consider that their animal 

 nature is yet proved beyond a doubt, though I have 

 myself once suggested the possibility of a definite relation 

 between them and the lowest Gasteropoda. 1 Since it 



1 Comp. Chap. I, Sect. 18, p. 113. 



U 



