206 ESSAY ON CLASSIFICATION 



tute classes. Moreover, the Helminths are linked to the Annelides in 

 the same manner as the apodal larvae of Insects are to the most 

 highly organized caterpillars. 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 Crus- 

 tacea and the Insects, respecting the limits of which as much has al- 

 ready 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 Ces- 

 tods, 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 com- 

 plicated 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 repro- 

 duction 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 cannot be sepa- 

 rated from the Placental Mammalia as a distinct class, since we ob- 

 serve, within the limits of another type of Vertebrata, 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 Mammalia. But I hold at the same time with other 

 naturalists that the Batrachia must be separated as a class from the 

 true Reptiles, as the characters which distinguish them are of the 



