INTRODUCTION. oS 
the energy of the circulating system, should then be the basis of the pri- 
mary divisions of the animal kingdom. We will afterwards ascertain, in 
each of these divisions, what characters should succeed immediately to those, 
-and form the basis of the primary subdivisions. 
General Distribution of the Animal Kingdom into four great Divisions. 
If, divesting ourselves of the prejudices founded on the divisions former- 
ly admitted, we consider only the organization and nature of animals, with- 
out regard to their size, utility, the greater or less knowledge we have of 
them, and other accessary circumstances, we shall find there are four prin- 
cipal forms—four general plans, if it may be so expressed, on which all ani- 
mals seem to have been modelled, and whose ulterior divisions, whatever 
be the titles with which naturalists have decorated them, are merely slight 
modifications, founded on. the developement or addition of certain parts, 
which produce no essential change in the plan itself, 
In the first of these forms, which is that of man, and of the animals most 
nearly resembling him, the brain and principal trunk of the nervous system 
are inclosed in a bony envelope, formed by the cranium and vertebre; to 
the sides of this intermedial column are attached the ribs, and bones of the 
limbs, which form the frame work of the body; the muscles generally cover 
the bones, whose motions they occasion, while the viscera are contained 
within the head and trunk. Animals of this form we shall denominate 
- Animalia Vertebrata. 
They have all red blood, a muscular heart, a mouth furnished with two 
jaws situated either above or before each other, distinct organs of sight, 
hearing, smell, and taste placed in the cavities of the face, never more than 
four limbs, the sexes always separated, and a very similar distribution of 
the medullary masses and the principal branches of the nervous system. 
By a closer examination of each of the parts of this great series of ani- 
mals, we always discover some analogy, even in species the most remote 
from each other; and may trace the gradations of one same plan from man 
to the last of the fishes. 
In the second form there is no skeleton; the muscles are merely at- 
tached to the skin, which constitutes a soft contractile envelope, in which, 
in many species, are formed stony plates, called shells, whose position and 
production are analogous to those of the mucous body. The nervous sys- 
tem is contained within this general envelope along with the viscera, and is 
composed of several scattered masses connected by nervous filaments; 
the chief of these masses is placed on the esophagus, and is called the brain. 
Of the four senses, the organs of two only are observable, those of taste 
and sight, the latter of which are even frequently wanting. One single 
