AN INTRODUCTION TO ZOOLOGY 



tinguish them from the four other classes which make up the 

 phylum Vertebrata, or animals possessing vertebral columns. 

 The scientific name of any animal consists of the terms used to 

 designate the genus and species; this is commonly followed by 

 the name of the zoologist who wrote the first authoritative de- 

 scription of that particular species. The scientific name of man 

 is therefore written Homo sapiens Linnaeus. 



The following concise synopsis of the chief groups of animals, 

 together with a brief characterization of each of the fifteen phyla, 

 will be of value to the student for subsequent reference. The 

 terms used in describing these subdivisions will not perhaps be 

 altogether understood by the beginner, but their meaning will 

 be made apparent in succeeding chapters. It will suffice to 

 say here that, in a general way, the classification of animals 

 depends chiefly upon the characteristics which are contrasted 

 in Table I. 



TABLE I 



CHIEF CHARACTERISTICS USED IN SEPARATING THE PHYLA OF ANIMALS 

 CONTRASTED 



1. Body composed of one microscopic 



unit, the cell. 



2. Body formed from two primary 



cell layers, i.e. diploblastic. 



3. Body radially symmetrical, i.e. 



parts (antimeres) radiating from 

 a central axis. 



4. Digestive cavity with only one 



opening. 



5. A cavity, the ccelom between the 



digestive tube and the body wall. 



6. Body segmented, i.e. composed of 



a lineal series of parts, called 

 metameres or somites. 



Body composed of many cells. 



Body formed from three primary cell 



layers, i.e. triploblastic. 

 Body bilaterally symmetrical, i.e. 



parts arranged in pairs on either 



side of a central plane. 

 Digestive cavity with two openings, 



the mouth and anus. 

 No ccelom present. 



Body unsegmented. 



The figures following the diagnosis of each phylum indicate 

 the number of recent species; those in parentheses, the number of 

 fossil species known to represent it. 



