NERVOUS SYSTEM OF ARTICULATA. 541 



Many facts, indeed, tend to show that each 

 segment of the body of articulated animals, of 

 an annular structure and cylindric form, such as 

 the long worms and the myriapoda, has in many 

 respects an independent sensitive existence, so 

 that when the body is divided into two or more 

 parts, each portion retains both the faculty of 

 sensation, and the power of voluntary motion. 

 As far as we can judge, however, the only ex- 

 ternal sense which is capable of being exercised 

 by this simple form of nervous system, is that of 

 touch ; all the higher senses evidently requiring 

 a much more developed and concentrated orga- 

 nization of nervous ganglia. 



In this division of the animal kingdom, the 

 primary nervous cords always pass along the 

 middle of the lower surface of the body, this 

 being the situation which, in the absence of a 

 vertebral bony column, affords them the best 

 protection. They may be considered as ana- 

 logous to the spinal marrow, and as serving to 

 unite the series of ganglia, through which they 

 pass, into one connected system. On arriving 

 at the oesophagus, they form round it a circle, or 

 collar, studded with ganglia, of which the up- 

 permost, or that nearest the head, is generally of 

 greater size than the rest, and is termed the 

 cesophageal, cephalic, or cerebral ganglion, being 

 usually regarded as analogous to tlie brain of 

 larger animals. Perhaps a more correct view of 



