MANUAL OF ELEMENTARY ZOOLOGY 



sometimes known together as the brain, (2) two slender 

 circumpharyngeal commissures running from these 

 System? round the pharynx, and (3) a ventral nerve cord 



which starts from the commissures between the 

 third and fourth segments and runs the whole length of 

 the body in the ccelom below the gut, swelling into a 

 ganglion in each segment. The first of these ganglia is 

 bilobed and is known as the subpharyngeal ganglion. 

 Nerves are given off to the prostomium from the supra- 



pharyngeal ganglia, and 

 sup. ph. a. /C / I ."\ t0 tne fi rst two segments 



from the commissures, 

 and the ventral cord 

 gives off three pairs of 

 nerves in each segment. 

 The forepart of the ali- 

 mentary canal receives 

 nerves from the cir- 

 cumpharyngeal com- 

 missures. Though the 

 ventral cord appears to 

 be single, it is really 

 double, and can be seen 

 in transverse sections to 

 be divided into right 



Fig. 139.— A diagram of the forepart of and left halves by con- 



the nervous system of the earthworm. nective tissue. Trans- 



c.fih.c, Circumpharyngeal commissure ; «., verse Sections alsO show 

 nerves; j>h., pharynx cut through; Sep., .. , ., ,, , 



septa; subph.g., subpharyngeal ganglia; tnat tne middle and 



gfngufofve^rafco a rT geal ""**'' "^ U PP er P art ° f the C0 ^ 



consists of fine longi- 

 tudinal nerve fibres and the lower and outer parts of nerve 

 cells. Above the mass of fine longitudinal fibres are three 

 bundles of such fibres, each bundle being enclosed in a 

 sheath and known as a giant fibre. The nerves consist of 

 afferent fibres, which start in sense cells of the epidermis 

 and end as a bunch of fibrils in the central nervous system, 

 and efferent fibres, which start from nerve cells in the 

 ganglia and end against muscle and other cells. 



An earthworm has no well-developed organs of sense, but 

 certain of the columnar cells of the epidermis are rod-shaped 



