THE PRINCIPLES OF BIOLOGY 9 



the deer family, and several other families. These are all 

 placed together in the next higher systematic unit called 

 an order, in this case, the order of ruminants. 



The ruminants, because they are covered with hair 

 and nourish the young with milk, are in every essential 

 respect related to the one-toed horses, the beasts of 

 prey, the apes, etc. Hence they are all placed in a 

 more inclusive division of animals, the class called 

 mammals. 



All mammals have the skeleton, or support of the 

 body, on the inside, the axis of which is called the verte- 

 bral column. This feature also belongs to the classes 

 of reptiles, amphibians, and fishes. It is therefore 

 consistent to unite these classes by a general idea or 

 conception into a great branch of animals called the 

 vertebrates. 



Returning from the general to the particular by succes- 

 sive steps, state the branch, class, order, family, genus, 

 and species to which the cow belongs. 



The Eight Branches or Sub-kingdoms. — The simplest 

 classification divides the whole animal kingdom into 

 eight branches, named and characterized as follows, be- 

 ginning with the lowest : I. Protozoans. One-celled. 

 II. Sponges. Many openings. III. Polyps. Circular; 

 cup-like ; having only one opening which is both mouth and 

 vent. IV. Echinoderms. Circular ; rough-skinned ; two 

 openings. V. Mollusks. No skeleton ; usually with ex- 

 ternal shell. VI. Vermes. Elongate body, no jointed legs. 

 VII. Arthropods. External jointed skeleton; jointed 

 legs. VIII. Vertebrates. Internal jointed skeleton with 

 axis or backbone. 



