MODERN SYSTEMS.

289

Rhizopoda produce any eggs, I shall have more to saypresently. As to the arrangement of the leading groups,Vertebrata , Articulata, Cephalopoda , Mollusca , Vermes,Radiata, 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 thewhole series of Mollusks between the types of Articulataand 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 themanifold relations existing between all animals. Indeed,no system can be true to nature which is based upon theconsideration of a single part or a single organ.

After these general remarks, I have only to show morein detail why I believe that there are only four greatfundamental groups in the animal kingdom, neither morenor less.

With reference to Protozoa , first, it must be acknow-ledged, that, notwithstanding the extensive investigationof modem writers upon Infusoria and Rhizopoda , the truenature of these beings is still very little known. TheRhizopoda have been wandering from one end of theseries of Invertebrata to the other, without finding aplace generally acknowledged as expressing their trueaffinities. The attempt to separate them from all theclasses with which they have been so long associated, andto 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 animalnature is yet proved beyond a doubt, though I havemyself once suggested the possibility of a definite relationbetween them and the lowest Gasteropods . 1 Since it

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

U