4 INTRODUCTION TO ZOOLOGY 



express what seem to be the natural relationships of the 

 animals classified, so far as we can at present determine them 

 with our still imperfect knowledge of the course of evolution, 

 which has resulted in all the manifold forms of life on the 

 earth. 



The twelve primary divisions, or phyla, of the animal 

 kingdom at present recognised are : 



I. Protozoa (unicellular microscopic forms). 

 II. Coelenterata (Hydra, Jelly-fish, Sea-anemones, etc.). 



III. Porifera (Sponges). 



IV. Echinodermata (Starfish, Sea-urchin, etc.), 

 V. Annelida (Ringed Worms, e.g. Earthworm). 



VI. Platyhelminthes (Flat-worms, Liver-fluke, etc.). 

 VII. Nematoda (round unsegmented Worms). 

 VIII. Polyzoa 1 (Sea-mats, e.g. Flustra, Bugula, etc.). 

 IX. Rotifera (Wheel Animalcules). 

 X. Mollusca (Snails, Mussels, etc.). 

 XL Arthropoda (Crustaceans, Insects, and Spiders). 

 XII. Chordata (all vertebrate animals). 



In the following pages, types of all these phyla will be 

 described, with the exception of the last, the Chordata, the 

 phylum which includes all Fishes, Amphibia, Reptiles, Birds, 

 and Mammals, such a vast and important assemblage of forms 

 that they are left for fuller treatment than is possible in this 

 volume. 



1 The Polyzoa should perhaps be united with the Brachiopods, the Lamp- 

 shells, in a phylum Molluscoidea, but in this book only the Polyzoa are 

 described. 



