10 INTRODUCTION 



The following estimates have been made of the number of the 

 described species in each phylum: 



Phylum I. Protozoa 8,000 



Phylum II. Coelenterata 7,000 



Phylum III. Vermes 9,000 



Phylum IV. Annelida 4,000 



Phylum V. Arthropoda 400,000 



Phylum VI. Mollusca 61,000 



Phylum VII. Echinodermata 4,000 



Phylum VIII. Chordata 37,000 



Key to the phyla of the Animal Kingdom: 



Ox Single-celled animals, aquatic and microscopic 1. Protozoa 



«2 Many-celled animals. 

 6i Body radially symmetrical. 



Ci Body with 2, 4, 6 or more, or without, definite radii 2. Ccelenterata 



Ca Body with 5 radii 7. Echinodermata 



62 Body bilaterally symmetrical. 

 Ci Respiratory organs not pharyngeal. 

 di Body without a calcareous shell (with rare exceptions). 



Ci Body not externally segmented (except in tapeworms) 3. Vermes 



62 Body externally segmented. 



/i No segmented locomotory appendages 4. Annelida 



/z Paired segmented appendages present 5. Arthropoda 



dj Body with a calcareous shell 6. Mollusca 



Ca Respiratory organs internal and pharyngeal 8. Chordata 



