ElaST BRANCHES 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, Ccelenterata, Echinodermata, Vermes, Mol- 

 lusca, Arthropoda, and Verteirata. Their approximate 

 relationships may be provisionally expressed by the follow- 

 ing 



Tabttlab View of the Eight Branches of the Animal Kingdom. 



VIII. Vertebrata. 

 Ascidians to Man, 



VII. Arthropoda. 

 Crustaceans and Insects. 



VI. MoUusca. 

 Clams, Snails, Catties. 



V. Vermes. 

 Flat and Round Worms, Polyzoa, 

 BracMopods, Annelids. 



I 



IV. Echinodermata. 

 Crinoids, Starfish, etc. 



III. Ccelenterata. 

 Hydra, Jelly-fishes. 



II. Porifera. 

 Sponges. 



Metazoa. 

 Many-celled animals, with 3 cell-layers. 



I. Pkotozoa. 



Single-celled animals. 



It should be understood by the student that the classifi- 

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

 cur present knowledge of the structure of the leading types 



