32 LNTROUUCTION. 



functions. Now, \vc see tliese two systems degrade and disappear togetlier. In 

 tlie lowest of animals, where the nerves cease to he visihle, there are no Ioniser 

 distinct fihres, and the organs of digestion are simply excavated in the homogeneous 

 mass of the body. In insects, the vascular system disappears even before the nervous 

 one ; but, in general, the disjiersion of the medullar)- masses accompanies that of the 

 muscular agents ; a spinal chord, on wliich the knots or ganglions represent so 

 manv brains, corresponds to a body divided into numerous rings, and supported by 

 pairs of members distributed along its length, &c. 



This correspondence of general forms, which residts from the arrangement of the 

 organs of motion, the distrihution of the nervous masses, and the energy of the circu- 

 lating svstem, should serve then for the basis of the primary sections to be made in 

 the animal kingdom. We will afterwards ascertain, in each of these sections, what 

 characters should succeed innnediately to these, and form the basis of the primary 

 subdivisions. 



GEXEKAL DISTRIBUTION' OF THE .VNIMAL KIXGD03I IXTO FOUR GREAT DIVISIONS. 



If the animal kingdom be considered with reference to the princijiles which we have 

 laid down, and, divesting ourselves of the prejudices founded on the divisions 

 formerly admitted, we regard only the organization and nature of animals, and not 

 their size, utility, the more or less knowledge which we have of thern, nor any 

 other accessory circumstances, it will be found that there exist four principal forms, 

 four general plans, if it may be thus expressed, on which all animals appear to have 

 been modelled, and the ulterior divisions of which, under whatever title naturalists 

 may have designated thern, are merely slight modifications, founded on the developje- 

 ment 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 which most 

 resemble him, the brain and the principal trunk of the nervous system are inclosed in 

 a bony envelojie, which is formed by the cranium and the vertebrex' : to the sides of this 

 medial column are attached the ribs, and the bones of the limbs, which compose the 

 framework of the body : the muscles generally cover the bones, the motions of \vhicii 

 they produce, and the viscera are contained within the head and trunk. Animals of 

 this form we shall denominate 



VERTEBRATE ANIMALS (.tiiiinolia veiichralci) . 



Tliey have all red blood, a muscular heart, a mouth furnished with two jaws, 

 placed one either before or aljove the other, distinct organs of sight, hearing, smell, 

 and taste, situated in the cavities of the face ; never more than four limbs ; the i 

 sexes always sejiarated ; and a very similar distribution of the medullary masses, and ! 

 of the principal branches of the nervous system. I 



On examining each of the parts of this great series of animals more closely, there ! 

 may always be detected some analogy, even in those species which are most remote 

 from one another ; and the gradations of one single plan may be traced from man to 

 the last of fishes. j 



In the second form there is no skeleton ; the muscles are attached only to the skiu, ' 



