vr NEMATODA NERVOUS SYSTEM I2/ 



The Nervous System. The central organ of the nervous 

 system is the circumoesophageal ring which surrounds the 

 pharynx, close to the anterior end of the body, in A. megalo- 

 cephala 1^ to 2 mm. behind the mouth. 1 Ganglion cells are 

 found in the ring, but they are not numerous, and are chiefly 

 aggregated round the points of origin of the nerves. 



Six short nerves, three on each side of the median line, run 

 forward from the ring, a pair of these ending in each of the three 

 papillae which surround the mouth. 



Behind, the nerve-ring gives off six main nerve trunks, of 

 which the dorsal and ventral nerves are usually the largest. 



FIG. 63. Diagram of the nervous system at the two ends of the body in Ascaris 

 vtegalocephala Cloq., <J. (After Hesse.) a, Circumoesophageal nerve-ring ; b, opening 

 of excretory ducts ; c, dorsal nerve ; d, dorso-lateral nerve ; e, ventro-lateral nerve 

 becoming the bursal nerve posteriorly ; /, the ventral nerve ; g, cloacal opening ; 

 h, sub-cuticular nerves running from c to/; k, spicules. 



These run in the median dorsal and ventral thickenings of the 

 sub-cuticular tissue, and are connected one with another by 

 numerous fine lateral branches running through the sub-cuticle. 



The lateral nerves, which consist of two or four bundles, one 

 or two lying dorsal and one or two ventral to each excretory 

 canal, have a double origin. The dorsal branches arise directly 

 from the nerve-ring, and at their point of origin there is a con- 

 siderable accumulation of ganglion cells, from which two com- 

 missures on each side run into the ventral nerve (Fig. 63,/). 

 The ventral branches arise from the ventral nerve-cord imme- 

 diately in front of the excretory pore. At the posterior end 

 the lateral nerves pass into the two branches into which the 

 ventral nerve dividea Just before the point where the ventral 

 1 B. Hesse, Zcitsc.hr. iciss. ZooL Bd. liv. 18^2, p. 548. 



