INTRODUCTION. Xi 



organs 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 arrangement 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 

 creation. If we divest ourselves of the prejudices founded on 

 the divisions formerly admitted, 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 may be the titles which naturalists 

 have given 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 

 inclosed in a bony envelope, formed by the cranium and ver- 

 tebrae ; 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 [comprising mammals,* 



* As the number of feet is immaterial in classification, and as the terms 

 quadruped and tetrapod have already led to a most incongruous association of 



