PRINCIPLES OF CLASSIFICATION 19 



some question as to the number of these different types, 

 but we are not far out of the way in recognizing seven 

 principal ones. These give rise to the seven principal 

 branches of the animal kingdom : Protozoa, Ccelenterata, 

 Mollusca, Yermes, Arthropoda, Echinodermata, and Chor- 

 data (which includes vertebrates). The followers of Cuvier 

 and Agassiz reduced these to four or five : Protozoa, Kadi- 

 ata, Mollusca, Articulata, and Vertebrata ; but a more thor- 

 ough knowledge of the different groups makes the larger 

 number preferable, the radiates and the articulates being 

 each divided into two. Many zoologists , break up the 

 Vermes into several distinct branches. 



The branches are again divided into classes. Thus the 

 mammals, birds, reptiles, amphibians, fishes, lampreys, and 

 lancelets are classes of vertebrates. The insects form a 

 class of Arthropods. 



Each class is again divided into orders. The Glires or 

 rodents, the gnawing animals, of which squirrels, mice, and 

 rabbits are examples, form an order of mammals. The 

 hoofed animals, Ungulata, form another, and each of these 

 again contains many families. 



Intermediate divisions are sometimes recognized, with 

 the prefixes super and sub. A subfamily is a division of a 

 family including certain genera. A superfamily is a group 

 of related families within the limits of an order. 



The red squirrel belongs to the branch Chordata, class 

 Mammalia, order Glires, family Sciuridae, genus .Sciurus, 

 species Hudsonicus. 



20. Trinomial names. Trinomial names are those in 

 which the binomial name of a species is followed by a sec- 

 ond adjective. These indicate subspecies or varieties con- 

 nected with geographical distribution. Thus many forms 

 have a northern variety, a southern variety, one in the 

 mountains, one on the plains, in the forests, or in other 

 peculiar situations. 



Thus the gray squirrel, typically southern, has a sub- 



