HISTORY OF ZOOLOGY. 21 



theory of Max Schultze. In the cell theory a simple or- 

 ganic principle was found for all living creatures, for highly 

 and for lowly organized plants and animals, and the wide 

 realm of Histology was laid open for scientific considera- 

 tion. 



REFORM OF THE SYSTEM. 



Foundation of Modern Zoology. With the establish- 

 ment of Comparative Anatomy and Embryology and the 

 application of these to systemization, and with the develop- 

 ment of the cell theory and the scientific idea of tissues, 

 which stands in connection with it, we may say that the 

 foundation of Zoology was laid. Wonderful advances 

 were made in the realm of vertebrate anatomy through the 

 classic researches of Owen, Johannes M tiller, Rathke, 

 Gegenbaur, and others; our conceptions of organization 

 have been completely reformed by the work of Dujardin, 

 Max Schultze, Haeckel, and others, who have proven the 

 unicellularity of the lowest animals. The germ-layer 

 theory was further elaborated by Remak and Kolliker; and 

 by Kowalewsky, Haeckel, and Huxley applied to the in- 

 vertebrated animals. It is beyond the limits of this brief 

 historical summary to go into what has been accomplished 

 in regard to the other branches of the animal kingdom ; it 

 must here be sufficient to mention the most important 

 reforms which the Cuvierian system has experienced under 

 the influence of increasing knowledge. 



The Division of the Radiata. Of the four types of 

 Cuvier undoubtedly the branch Radiata was the one of 

 whose representatives he had the least knowledge ; it was 

 therefore naturally least condensed, since it comprised, be' 

 sides the radially symmetrical coelenterates and echino- 

 derms, other forms, which like the worms were bilaterally 

 symmetrical, or like many infusorians were asymmetrically 

 built. Thus it came about that most reforms have here 

 found their point of attack. 



C. Th. v. Siebold was the originator of the first im- 

 portant reform. He limited the type Radiata, or, as he 



