PRINCIPAL SYSTEMS OF ZOOLOGY 217 



Third Branch. Animai.ia Articulata. 



Cl. 1. Annelides. Ord. Tubicolas, Dorsibranchiac, Abranchiae. 



Cl. 2. Crustacea. 1st Section: Malacostraca. Ord. Decapoda, Stomapoda, Amphipoda, 



Lacmodipoda, Isopoda. 2d Section: Entomostraca. Ord. Branchiopoda, Poecilo- 



poda, Trilobitae. 

 Cl. 3. Arachnides. Ord. Puhnonariae, Tracheariae. 

 Cl. 4. Insects. Ord. Myriapoda, Thysanura, Parasita, Suctoria, Coleoptera, Orthoptera, 



Hemiptera, Neuroptera, Hymenoptera, Lepidoptera, Rhipiptera, Diptera. 



Fourth Branch. Animalia Radiata. 



Cl. 1. Echinoderms. Ord. Pedicellata, Apoda. 



Cl. 2. Intestinal Worms. Ord. Nematoidea (incl. Epizoa and Entozoa) Parenchyraatosa. 



Cl. 3. Acalephae. Ord. Simplices, Hydrostaticas. 



Cl. 4. Polypi. (Including Anthozoa, Hydroids, Bryozoa, Corallinae, and Spongia;). 



Ord. Carnosi, Gelatinosi, Polypiarii. 

 Cl. 5. Infusoria. Ord. Rotifera and Homogenea (including Polygastrica and some 



Algae.) 



When we consider the zoological systems of the past century, that 

 of Linnaeus, for instance, and compare them with more recent ones, 

 that of Cuvier, for example, we cannot overlook the fact that even 

 when discoveries have added little to our knowledge, the subject is 

 treated in a different manner; not merely in consequence of the more 

 extensive information respecting the internal structure of animals, 

 but also respecting the gradation of the higher groups. 



Linnseus had no divisions of a higher order than classes. Cuvier 

 introduced for the first time four great divisions, which he called 

 " embranchements" or branches, under which he arranged his classes, 

 of which he admitted three times as many as Linnaeus had done. 



Again, Linnzeus divides his classes into orders; next, he introduces 

 genera, and finally, species; and this he does systematically in the 

 same gradation through all classes, so that each of his six classes is 

 subdivided into orders, and these into genera with their species. Of 

 families, as now understood, Linnaeus knows nothing. 



The classification of Cuvier presents no such regularity in its 

 framework. In some classes he proceeds, immediately after present- 

 ing their characteristics, to the enumeration of the genera they con- 

 tain, without grouping them either into orders or families. In other 

 classes he admits orders under the head of the class, and then proceeds 

 to the characteristics of the genera, while in others still, he admits 

 under the class not only orders and families, placing always the fam- 

 ily in a subordinate position to the order, but also a number of sec- 



