GENERAL CHARACTERISTICS. 3 



the Hoofed Animals or Ungulates (pigs, deer, cattle, horses, etc.), and the 

 Carnivores (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. Con- 

 sequently 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 commonly 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 vertebral, and the whole column as the vertebral column; 

 while all the classes possessing this vertebral column are grouped together under 

 the name of Vertebrates, or Vertebrata, this largest group being known as a sub- 

 kingdom. 



We accordingly have a scheme of classification like the following : 



Subkingdom VERTEBRATA, or Vertebrates. 

 Class Mammalia, or Mammals. 

 Order RODEXTIA, or Rodents. 



Family MURID^E, or Rats and Voles. 

 Genus Mus, Rats and Mice. 

 Genus Microtus, Voles. 



structure I n sa yi n g that the Vertebrates, or highest of all animals, are 



of the characterised by the presence of a backbone or vertebral column, we 

 jrates. nave gi ven on iy 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 back- 

 bone lying nearest to the back there runs a tube or canal, formed by arches of bone 

 or cartilage springing from the bodies of the vertebrae, within which tube is the 

 so-called spinal marrow 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. 



