20 INTEODUCTION. 



functions. Now, we see these two systems degrade and disappear together. In 

 the lowest of animals, where the nerves cease to be visible, there are no longer 

 distinct fibres, 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 dispersion of the medullary masses accompanies that of the 

 muscular agents : a spinal chord, on which the knots or ganglions represent so 

 many 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 results from the arrangement of the 

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

 lating system, 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 immediately to these, and form the basis of the primary 

 subdivisions. 



GENERAL DISTRIBUTION OF THE ANIMAL KINGDOM INTO FOUR GREAT DIVISIONS. 



If the animal kingdom be considered with reference to the principles 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 them, 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 them, are merely slight modifications, founded on the develope- 

 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 envelope, which is formed by the cranium and the vertebrae : to the sides uf 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 which 

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

 this form we shall denominate 



VERTEBRATE ANIMALS (A/imalia vcrtehrattt). ', 



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

 placed one citlicr before or above the other, distinct organs of sight, hearing, smell, 

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

 SCXC9 always separated ; and a very similar distribution of the medullary masses, and 

 of the principal branches of the nervous system. 



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 fislies. 



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



