INTRODUCTION 7 



pairs of limbs. They thus differ in some of the most funda- 

 mental features of their organisation from such animals as 

 crabs, insects, scorpions, and centipedes, which have colour- 

 less blood, a jointed external skeleton, and numerous limbs. 

 These differences — far greater than those between classes 

 — are expressed by placing the backboned animals in 

 the phylum or sub-kingdom Cho?-data, the many-legged 

 armoured forms in the phylum Arthropoda. Similarly, soft- 

 bodied animals with shells, such as oysters and snails, form 

 the phylum Mollusca, polypes and jellyfishes the phylum 

 Ccelenterata. And, finally, the various phyla recognised by 

 zoologists together constitute the kingdom Animalia. 



Thus the animal kingdom is divided into phyla, the phyla 

 into classes, the classes into orders, the orders into families, 

 the families into genera, and the genera into species, while 

 the species themselves are assemblages of individual animals 

 agreeing with one another in certain constant characters. It 

 will be seen that the individual is the only term in the series 

 which has a real existence : all the others are mere groups 

 formed, more or less arbitrarily, by man. 



To return to the animal originally selected as an example, 

 it will be seen that the zoological position of the domestic 

 cat is expressed as follows : — 



Kingdom — Animalia. 



Phylum — Chordata. 

 Class — Mammalia. 



Order — Carnivora. 

 Family — Felid<x. 

 Genus — Felis. 



Species — F. domes tica. 



The object of systematic zoologists has always been to 



