186 ESSAY ON CLASSIFICATION. Part I. 



organization.^ In bringing these animals together, naturalists make again the same 

 mistake which Cuvier committed, when he associated the Helminths with the 

 Radiates, only in another way and upon a greater scale.^ The Bryozoa are as it 

 were depauperated Mollusks, as Aphanes and Alchemilla are depauperated Eosacese. 

 Rotifera are in the same sense the lowest Crustacea ; while Helminths and Annelides 

 constitute together the lowest class of Articulata. This class is connected by the 

 closest homology with the larval states of Insects ; the plan of their structure is 

 identical, and there exists between them only such structural differences as con- 

 stitute classes.^ Moreover, the Helminths are linked to the Annelides in the same 

 manner as the apodal larvEe of Insects are to the most highly organized cater- 

 pillars. It may truly be said that the class of Worms represents, in perfect animals, 

 the embryonic states of the higher Articulata. The two other classes of this 

 branch are the Crustacea and the Insects, respecting the limits of which, as much 

 has already been said above,* as is necessary to state here. 



The classification of the branch of Articulata may, therefore, stand thus : — 



1st Class: Worms; with three orders, Trematods, (including Cestods, Planariae, 

 and Leeches,) Nematoids, (including Acanthocephala and Gordiacei,) and Annelides. 



2d Class: Crustacea; with four orders, Rotifera, Entomostraca, (including 

 Cirripeds,) Tetradecapods, and Decapods. 



3d Class : Insects; with three orders, Myriapods, Arachnids, and Insects 

 proper. 



There is not a dissenting voice among anatomists respecting the natural limits 

 of the Vertebrata, as a branch of the animal kingdom. Their character, however, 

 does not so much consist in the structure of their backbone or the presence of 

 a dorsal cord, as in the general plan of that structure, which exhibits a cavity 

 above and a cavity below a solid axis. These two cavities are circumscribed by 

 complicated arches, arising from the axis, which are made up of different systems 

 of organs, the skeleton, the muscles, vessels, and nerves, and include, the upper 

 one the centres of the nervous system, the lower one the different systems of 

 organs by which assimilation and reproduction are carried on. 



The number and limits of the classes of this branch are not yet satisfactorily 

 ascertained. At least, naturalists do not all agree about them. For my part, I 

 believe that the Marsupialia cainiot be separated from the Placental Mammalia, 

 as a distinct class, since we observe, within the limits of another type of Vei'te- 

 brata, the Selachians, which cannot be subdivided into classes, similar differences in 

 the mode of development to those which exist between the Marsupials and the other 



1 See above, Chap. I., Sect. 18, p. 74-78. ' Compare Chap. II., Sect. 2, p. 145. 



^ Compare Chap. II., Sect. 1, p. 142. * Compare Chap. I., Sect. 18, p. 78-80. 



