GENERAL CHARACTERISTICS 3 



vores (cats, dogs, etc.), which form three distinct orders, all agree with one another 

 in that their young are nourished by milk sucked from the mother. Consequently 

 such animals, together with all others showing the same peculiarity, are grouped 

 together to form the class of Mammals or Mammalia. 



If, however, a mammal, a bird, a reptile, and a fish be compared together, it 

 will be found that although the three latter differ from the former, in that the young 

 are not suckled by the female parent, yet all agree in the possession of what we com- 

 monly call the backbone; this backbone consisting of a column running along the 

 back of the animal, and composed of a number of jointed segments, which, although 

 usually formed of bone, may be of cartilage. Such joints are technically known as 

 vertebrcz, and the whole column as the vertebral column; while all the classes possess- 

 ing this vertebral column are grouped together under the name of Vertebrates, or 

 Vertebrata, this largest group being known as a subkingdom. 



We accordingly have a scheme of classification like the following : 



Subkingdom VERTEBRATA, or Vertebrates. 

 Class Mammalia, or Mammals. 

 Order RODENTIA, or Rodents. 



Family MURID^, or Rats and Voles. 

 Genus Mus, Rats and Mice. 

 Genus Microtus, Voles. 



Structure In saying that the Vertebrates, or highest of all animals, are character- 



ized by the presence of a backbone or vertebral column, we have given 

 only the primary feature of this great group; and we must accordingly 

 say a few words more on the subject of their structure. Now an essential feature 

 in the structure of all Vertebrates is that on that side of the backbone lying nearest 

 to the back there runs a tube or canal, formed by arches of bone or cartilage spring- 

 ing from the bodies of the vertebrae, within which tube is the so-called spinal mar- 

 row or cord, which is a rope-like structure formed of nerve-tissue, and running 

 backwards from the brain to the hinder extremity of the body. On the opposite 

 side of the backbone to that occupied by the spinal marrow there is a much larger 

 cavity containing the viscera, such as the heart, lungs, stomach, etc. In a cross- 

 section of the body of any vertebrate animal we therefore see two tubes a small 

 one containing the nervous system placed above the backbone, and a much larger 

 one containing the viscera situated below the backbone. 



Another noteworthy peculiarity of Vertebrates is that the limbs, which never 

 exceed four in number, are always directed away from that part of the body which 

 contains the nervous system, and towards that enclosing the viscera; whereas in 

 nearly all the lower animals, collectively known as the Invertebrates, the reverse is 

 the case. Vertebrates are likewise distinguished by the circumstance that the two 

 jaws work in a vertical plane, or, in other words, are upper and lower, instead of 

 being right and left, as they are in insects. 



Having said thus much as to the general characters of the Vertebrate sub- 

 kingdom, we come to the consideration of those of its highest class, the Mammals. 



