296 BIOLOGY: GENERAL AND MEDICAL 



Cycle III. Respirative animals 



7. Worms (skin animals) 



8. Crustacea (branchial animals) 



9. Insects (tracheal animals) 



A new idea followed the active pursuit of embryology 

 by the German naturalists and is expressed by von 

 Baer, whose idea that "ontogeny recapitulates phy- 

 logeny" led him to propose the following classification 

 based upon the embryological development of the mem- 

 bers of the various groups: 



I. Peripheral type (Radiata) 

 II. Massive type (Mollusca) 



III. Longitudinal type (Articulata) 



IV. Doubly symmetrical type (Vertebrata) 



"Thus, von Baer, with his classification, based on 

 embryological principles, and Cuvier, with his, founded 

 on comparative anatomy, arrived at very similar con- 

 clusions, viz. : that animals are built upon four general 

 plans and fall into four general groups. In the end, the 

 system of Cuvier triumphed over that of the natural 

 philosophers." 



Revisions of the classification of the invertebrata 

 were published by Agassiz and later by Huxley and did 

 much to assist in promoting research upon the animals 

 of these divisions. As, however, their classifications 

 were not based upon any essentially new idea, it seems 

 proper to pass on to the modern classification. 



According to the plan adopted at present, the whole 

 animal kingdom is divided into two Sub-divisions, 

 the Protozoa and the Metazoa. Each sub-division is 

 made up of certain grand groups or Phyla which repre- 

 sent more or less well-marked plans of structure, and 

 form the points about which all the animals constructed 

 upon a similar general plan are arranged. Each phylum 

 includes a number of Classes, each of which is composed 

 of organisms which, though phyletically related, differ 

 in some constant feature, such as having six legs or 



