1 88 ANIMALS 



f erentiation of the elements, but the more significant difference 

 is the centraHzation. The brain of the worm has no counter- 

 part in the medusae where there are a number of centres of co- 

 ordinate rank. 



443. The central nervous system of Arthropods is, in most 

 respects, much hke that of Worms. The most significant 

 difference is the better development of the brain, which results 

 largely from the better development of the special sense organs. 

 The differentiation of body segments and of the appendages 

 also makes possible a much greater number of movements, and, 

 therefore, demands greater complexity in association elements 

 of the nervous system. Besides, the nervous system is itself 

 subject to independent differentiation, and this manifests 

 itself in the increased complexity of the association elements. 

 This takes place especially in the brain or highest centre, and 

 it is in this respect especially that the brain of the crayfish 

 and insect is in advance of that of the worm. 



444. The central nervous system of the Vertebrates is wholly 

 dorsal and consists of the brain and spinal cord. It originates 

 from the ectoderm as a tube, but the cavity of the tube remains 

 extremely small, except in the region of the brain, where a series 

 of chambers of considerable size are developed. The nerve 

 elements develop in the walls of this tube and form a continu- 

 ous ganglionic mass. The nerves are arranged segmentally, 

 but there is little evidence of segmentation in the brain or spinal 

 cord. In a cross section of the spinal cord the nerve cells are 

 seen to be massed in the central portion in an area resembling 

 a letter H. The space around this is made up of longitudinal 

 nerve fibres, which are each one encased in a thick sheath of a 

 fatty substance, which gives them an opaque white appearance 

 when seen in mass. The central gray area contains naked 

 fibres as well as cells. In the central parts of the brain there are 

 also large masses of the cellular gray matter, but a still larger 

 quantity of the gray is distributed over the surfaces of the folded 



