156 GENERAL ZOOLOGY 



phyla of animals was, therefore, probably the same up to 

 the gastrula stage (with one exception, and this will be 

 considered in the next chapter). 



Aside from the development of the ccelom, the most 

 important advances in the recent racial history of animals 

 are: (1) The ascendency of the bilateral type of symmetry. 

 (2) The development of paired appendages. (3) The more 

 "progressive" bilaterally symmetrical animals are all 

 metameric, and the simpler metameric types consist of a 

 similar series of segments from the anterior to the posterior 

 end. This is homonomous metamerism, such as is present 

 in the earthworm. (4) In the most specialized phyla 

 of metameric animals, however, there is more importance 

 given to the segments toward the anterior end (i.e.j there is 

 progressive cephalization), and this often results in the 

 formation of a distinct head which bears important sense 

 organs. Appendages develop first, both phylogenetically 

 and ontogenetically,* as mere outgrowths or flaps from the 

 body and may later become jointed or modified in various 

 ways. 



* Phylogeny is racial development; ontogeny is individual development. 



