MODERN SYSTEMS. 295 



have been clearly demonstrated by the investigations of 

 Milne-Edwards. Vogt is the only naturalist who con- 

 siders the Cephalopoda " as built upon a plan entirely 

 peculiar"; 1 though he does not show in what this pecu- 

 liarity of plan consists, but only mentions the well-known 

 anatomical differences which distinguish them from, the 

 other classes of the branch of Mollusks. These differ- 

 ences, however, constitute only class characters, and 

 exhibit in no way a different plan. It is, indeed, by no 

 means difficult to honiologize all the systems of organs of 

 the Cephalopods with those of the other Mollusks, 2 and 

 with this evidence the proof is also furnished that the 

 Cephalopods constitute only a class among the Mollusks. 

 As to the differences in the development of the Cepha- 

 lopods and the other Mollusks, the type of Vertebrata 

 teaches us that partial and total segmentation of the 

 yolk is not inconsistent with unity of type, as the eggs 

 of Mammalia and Cyclostomata undergo a total segmenta- 

 tion, while the process of segmentation is more or less 

 limited in the other classes. In Birds, Keptiles, and 

 Selachians, the segmentation is only superficial ; in 

 Batrachians, and most Fishes, it is much deeper ; and 

 yet no one would venture to separate the Vertebrata into 

 several distinct branches on that account. With refer- 

 ence to Bryozoa, there can be no doubt that their asso- 

 ciation with Polypi or with Worms is contrary to their 

 natural affinities. The plan of their structure is in no 

 way radiate ; it is, on the contrary, distinctly and essen- 

 tially bilateral ; and as soon as their close affinities with 



1 VOGT (C.), Zoologische Briefe, Cephalous Mollusca, Tr. Roy. Soc. ; 

 q. a., vol. i, p. 361. London, 1853, p. 29. V. CARUS, 



2 See LEUCKART, Ueber die Mor- System der thierischeu Morphologic 

 phologie, etc., q. a., p. 24, note 2. q. a., p. 24, note 2. 



EUXLEY, On the Morphology of the 



