84 
PRINCIPAL TYPES OF ANIMAL STRUCTURE. 
CHAPTEK 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 Annual 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. Eor 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. 
7 0. 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 
