EIGHT BRAJSrCHES OF THE ANIMAL KINGDOM. 15 



idea of the complicated relations between animals of differ- 

 ent groups. 



The Animal Kingdom may be divided primarily into 

 two series of branches ; those for the most part composed 

 of a single cell, represented by a single branch, the Proto- 

 zoa, and those whose bodies are composed of many cells 

 {Metazoa), the cells arranged in three fundamental cell- 

 layers — viz., the ectoderm, mesoderm, and endoderm. The 

 series of Metazoa comprises the seven higher branches — i.e., 

 the Porifera, Cwlenterata, Echinodermata, Vermes, Mol- 

 lusca, Arthropoda, and Vertebrata. Their approximate 

 relationships may be provisionally expressed by the follow- 



Tabulab View op the Eight Branches op the Animal Kingdom. 



VIII. Vertebrata. 

 Fishes to Man. 



VII. Arthropoda. 

 Crustaceans and Insects. 



VI. MoUusca,, 

 Clams, Snails, C littles. 



V. Vermes, 

 yiat and Round Worms, Polyzoa, 

 Brachiopods, Annelids, Tunicates. 



I 



IV. Echinodermat.a. 

 Crinoids, Starfish, etc. 



III. Gmlenterata. 

 Hydra, Jelly-fishes. 



II. Porifera. 

 Sponges. 



Metazoa. 

 Many-celled animals, with 3 cell-layers. 



I. Protozoa. 



Single-celled animals. 



It should be understood by the student that the classifi- 

 cation presented in this book is a provisional one, based on 

 our present knowledge of the structure of the leading types 



