220 THE FROG chap. 



in the phylum Arthropoda. Similarly, soft-bodied animals 

 with shells, such as mussels and snails, form the phylum 

 Mollusca ; various worms, such as the earthworm, the phylum 

 Annulata; polypes and jelly-fishes the phylum Calenterata ; 

 the simplest animals, mostly minute, such as Amxba, the 

 phylum Protozoa. 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 

 definite characteristics. 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 arbi- 

 trarily, by man. 



Thus the zoological position of the common frog is 

 expressed as follows : — 



Kingdom — Animalia. 



Phylum — Vertebrata. 

 Class — Amphibia. 

 Order — Anura. 



Family — Ranida. 

 Genus — Rana. 



Species — temporaria. 



Let us now briefly consider some of the reasons which 

 have led zoologists to adopt the system of classification now 

 in general use. 



It is obvious that there are various ways in which animals 

 may be classified, and the question arises — which of these, 

 if any, is the right one ? Is there any standard by which 

 we can judge of the accuracy of a given classification, or 



