18 GENERAL PRINCIPLES OF ZOOLOGY. 



ciple of organization 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 treatment. 



KEFORM OF THE SYSTEM. 



Foundation of Modern Zoology. With the establishment of 

 comparative anatomy and embryology and the application of these 

 to classification, and with the development of the cell theory and 

 of histology, which is connected with it, we may say that the 

 foundation of zoology was laid. Wonderful advances were made 

 in vertebrate anatomy by the classic researches of Owen, Johannes 

 Miiller, Rathke, Gegenbaur, and others; our conceptions of organ- 

 ization have been completely altered by the work of Dujardin, 

 Max Schultze, Haeckel, and others, who have proved the unicellu- 

 larity of the lowest animals. The germ -layer theory was further 

 elaborated by Remak and Kolliker; and applied to the invertebrate 

 animals by Kowalewsky, Haeckel, and Huxley. 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 king- 

 dom; it must here be sufficient to mention the most important 

 changes which the Cuvierian system has undergone under the 

 influence of increasing knowledge. 



The Division of the Radiata. Of the four types of Cuvier the 

 branch Radiata was undoubtedly the one of whose representatives 

 he had the least knowledge; it was therefore the least natural, 

 since it comprised, besides the radially symmetrical coelenterates 

 and echinoderms, other forms, which, like the worms, were 

 bilaterally symmetrical, or, like many infusorians, were asym- 

 metrical. Thus it came about that most reforms have here found 

 their point of attack. 



C. Th. von Siebold was the originator of the first important 

 reform. He limited the type Radiata, or, as ho termed them, the 

 Zoophytes, to those animals with radially symmetrical structure 

 (Echinoderms and the Plant-animals) ; separating all the others, 

 he formed of the unicellular organisms the branch of ' primitive 

 animals' or Protozoa; the higher organized animals he grouped 

 together as worms or Vermes; at the same time he transferred a 

 part of the Articulata, the annelids, to the worm group, and pro- 

 posed for the other articulates, crabs, millipedes, spiders, and 

 insects, the term Arthropoda. 



Leuckart, about the same time (1848), divided the branch 



