84 PRINCIPAL TYPES OP ANIMAL STRUCTURE. 



CHAPTEE II. 



GENERAL VIEW OF THE ANIMAL KINGDOM. 



69. WHEN we examine the Animal Kingdom as a whole, it . 

 is easy to distinguish in it four general plans or types of struc- 

 ture, by which, with almost infinite variations in detail, the 

 formation of the several beings that compose it has been 

 guided. As specimens of these four plans or types, we may 

 name four animals which are familiar to almost every one, 

 the Dog, the Lobster, the Snail, and the Star-fish. The dif- 

 ferences by which these types are distinguished, are mani- 

 fested in the arrangement of the different organs of the body ; 

 and particularly in the form of the .nervous system and its 

 instruments. It has been already stated ( 4 ) that the power 

 of feeling, and of spontaneous motion, is that which peculiarly 

 distinguishes the Animal from the Plant; and as these powers 

 are possessed in very different degrees, and exercised in very 

 different modes, by the various tribes of animals, whilst the 

 operations' of -nutrition are performed, as in plants, in a much 

 more uniform manner, they afford us a satisfactory means of 

 separating these tribes from one another. For the nervous 

 system is the organ to which these powers are due ; and we 

 find it presenting forms so different in the four great divisions 

 already alluded to, that we can at once distinguish them by 

 this alone, even where (as sometimes happens) there may be 

 such a blending, in a particular animal, of the general characters 

 of two of them, as to lead us to hesitate in assigning its precise 

 place in the animal kingdom. 



70. The highest of these four divisions is that denominated 

 VERTEBRATA, or Vertebrated Animals; it receives its name 

 from the structure characteristic of it, the possession of a 

 jointed back-bone or vertebral column, which will be pre- 

 sently described. This is the group to which Man belongs ; 

 and all the animals it contains bear a greater or less resem 

 blance to him in structure. We notice in regard to their 

 external form, that they are alike on the two sides of their 

 body; every part having its fellow on the other side. This 



