24 INTRODUCTION. 



the vegetative functions, as the brain and the trunk of the nervous 

 system do for the animal ones. Now we see these two systems become 

 imperfect, and disappear together. In the lowest class of animals, where 

 the nerves cease to be visible, the fibres are no longer distinct, and the 

 organs of digestion are simple excavations in the homogeneous mass of 

 the body. In insects the vascular system even disappears before the 

 nervous one; but, in general, the dispersion of the medullary masses 

 accompanies that of the muscular agents : a spinal marrow, on which 

 the knots or ganglions represent so many brains, corresponds to a body 

 divided into numerous rings, supported by pairs of limbs longitudinally 

 distributed, &c. 



This correspondence of general forms, which results from the arrange- 

 ment of the organs of motion, the distribution of the nervous masses, 

 and the energy of the circulating system, should then be the basis of the 

 primary divisions of the animal kingdom. We will aftenvards ascertain, 

 in each of these divisions, what characters should succeed immediately 

 to them, and form the basis of the primary subdivisions. 



GENERAL DISTRIBUTION OP THE ANIMAL KINGDOM INTO FOUR 

 GREAT DIVISIONS. 



If, divesting ourselves of the prejudices founded on the divisions 

 formerly admitted, we consider only the organisation and nature of ani- 

 mals, without regard to their size, utility, the greater or less knowledge 

 we have of them, and other accessary circumstances, we shall find there 

 are four principal forms, four general plans, if it may be so expressed, 

 on which all animals 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 development 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 enclosed in a bony envelope, formed by the cranium 

 and vertebra ; 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. 



