260 



ESSENTIALS OF ZOOLOGY 



orders, and phyla of related classes. These groups are regarded as 

 having arisen from successively more remote ancestors. To the ex- 

 tent that evolution has proceeded in the different groups, this sys- 

 tem of classification carries a fairly accurate outline of the phylo- 

 genetic background of any modern animal. Forms possessing ho- 

 mologous structures (structures of similar form and similar em- 

 bryonic development) are described as of close relationship. Since 

 similarity in embryonic development is regarded as most important 

 evidence of homology, the theory of recapitulation (individual in its 

 life lives over the history of its race) enters strongly in determining 

 relationships of animals and animal groups. 



Chordata 

 (Frog) 



Annelida 

 {lumbhcus) 



Bryo^oa 

 (Bugulaj 



Rotifera 



Platyhdminthei 

 {Tapeworm) 



Arthropoda 

 (Crayfish) 



MoUusca 

 (Snail) 



Ecblnodermata 

 (Jtarfiih) 



Nemathelmintbe5 

 (Ascans) 



Per if era 

 (icypha) 



Coelenteraba 

 (Hydra) 



Ctenophora 



(Beroe j 

 sea walnut 



Protozoa 

 (ParameciurT)) 



ANIMAL KINGDOM 



Fig. 85. — Phylum relations in the animal kingdom. 



Rules of Nomenclature 



In order that each kind of animal may have a valid and recognized 

 scientific name, and not several different names, and also that two 

 different animals will not have the same name, some definite rules 

 of nomenclature have been organized into a code. The code which 

 is most generally used is the hit er national Code of Zoological Nomen- 

 clature. It was adopted by the International Zoological Congress and 

 is administered through a Commission on Nomenclature. 



