58 THE MAIN CURRENTS OF ZOOLOGY 



animals exhibit four types of organization (Verte- 

 brata, Mollusca, Articulata and Radiata) and his 

 types were substituted for the primary groups of 

 Linnaeus. 



But naturalists were not long in discovering that 

 the primary divisions of Cuvier were not well bal- 

 anced, and, indeed, that they were not natural 

 divisions of the animal kingdom. 



The group Radiata was the least sharply denned, 

 since Cuvier had included in it not only those animals 

 which exhibit a radial arrangement of parts, but also 

 unicellular animals that were asymmetrical, and 

 some of the worms that showed bilateral symmetry. 

 Accordingly, Karl Th. von Siebold, in 1845 separated 

 these animals and redistributed them. For the 

 simplest unicellular animals he adopted the name 

 Protozoa, which they still retain, and the truly 

 radiated forms, as starfish, sea-urchins, hydroid 

 polyps, coral animals, etc., were united in the group 

 Zoophyta. Von Siebold also changed Cuvier's 

 branch, Articulata, separating those forms, such as 

 Crustacea, insects, spiders, and myriopods, which 

 have jointed appendages, into a natural group called 

 Arthropoda, and uniting the segmented worms with 

 those worms that Cuvier had included in the radiate 



