PRINCIPAL SYSTEMS OF ZOOLOGY 199 



logical principles, though seemingly borrowed from that all im- 

 portant guide, Embryology. 



The divisions adopted by Leuckart are: Protozoa (though he does 

 not enter upon an elaborate consideration of that group), Coelenter- 

 ata, Echinodermata, Vermes, Arthropoda, Mollusca, and Vertebrata. 

 The classification adopted many years before by Siebold in his text- 

 book of comparative anatomy is nearly the same, except that Mol- 

 lusks follow the Worms, that Coelenterata and Echinoderms are 

 united into one group, and that the Bryozoa are left among the 

 Polyps. 



Here we have a real improvement upon the classification of Cuvier, 

 inasmuch as the Worms are removed from among the Radiates and 

 brought nearer the Arthropods, an improvement however, which, 

 so far as it is correct, has already been anticipated by many natural- 

 ists, since Blainville and other zoologists long ago felt the impropri- 

 ety of allowing them to remain among Radiates and have been in- 

 duced to associate them more or less closely with Articulates. But I 

 believe the union of Bryozoa and Rotifera with the Worms, proposed 

 by Leuckart, to be a great mistake; as to the separation of Coelen- 

 terata from Echinoderms, I consider it as an exaggeration of the dif- 

 ference which exists between Polyps and Acalephs on the one hand, 

 and Echinoderms on the other. 



The fundamental groups adopted by Vogt^ are: Protozoa, Radiata, 

 Vermes, Mollusca, Cephalopoda, Articulata, and Vertebrata, an ar- 

 rangement which is based solely upon the relations of the embryo to 

 the yolk, or the absence of eggs. But as I have already stated, this is 

 an entirely unphysiological principle, inasmuch as it assumes a con- 

 trast between the yolk and the embryo, within limits which do not 

 exist in nature. The Mammalia, for instance, which are placed, like 

 all other Vertebrata, in the category of the animals in which there is 

 an opposition between the embryo and the yolk, are as much formed 

 of the whole yolk as the Echinoderms or Mollusks. The yolk under- 

 goes a complete segmentation in Mammalia, as well as in Radiates 

 or Worms, and most Mollusks; and the embryo when it makes its 

 appearance no more stands out from the yolk than the little Starfish 



^ Carl Vogt, Zoologische Briefe. Naturgeschichte der lebenden und untergegangenen 

 Thiere (2 vols., Frankfurt a. M., 1851), I, 70. 



