178 INVERTEBRATE ZOOLOGY 



This subdivision of the animal kingdom is based upon one 

 made by Cuvier in the early part of the last century. He divided 

 animals into four main groups or " embranchements," which 

 were the following: 



I. Radiata (protozoans, jelly-fishes, starfishes, etc.). 



II. Articulata (annelids and arthropods). 



III. Mollusca (mollusks, brachiopods, cirripeds, etc.). 



IV. Vertebrata (vertebrates). 



The idea upon which Cuvier based his system was the fol- 

 lowing: that four distinct and different types of structure 

 appear in the animal kingdom; that all the members of each 

 " embranchement " present the peculiar structural features of 

 one of these types and may be arranged in an ascending series, 

 which begins with those animals having the simplest structure 

 and the least specialization of parts and ends with those pre- 

 senting the most complicated structure and the highest degree 

 of specialization. 



The first important improvements in this system were made 

 by von Siebold and Leuckart, who in 1845 and 1848 founded the 

 group Protozoa to include the single-celled animals, and further 

 divided the group Articulata into Vermes and Arthropoda and the 

 group Radiata into Echinodermata and Coelenterata. Claus further 

 improved it by adding the groups Molluscoidea and Tunicata, 

 bringing the total number of subkingdoms up to nine. 



With the firm establishment, however, of the theory of the 

 common descent and blood relationship of all animals by Darwin 

 in 1859, a new tendency began to manifest itself in the classifi- 

 cation of them. Ernst Haeckel, a professor in the University of 

 Jena, soon began to construct a system which should express 

 these relationships. The old system could not, however, be 

 easily given up. Claus, who was a professor in the University 

 of Vienna, opposed and antagonized Haeckel, and published in 

 his text-book his improvement of the old classification just 



