{127}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. megalocephala_ 1½ to 2 mm. behind the mouth.[157] 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. 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. [Illustration: FIG. 63.—Diagram of the nervous system at the two ends of the body in _Ascaris megalocephala_ Cloq., ♂. (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; _f_, the ventral nerve; _g_, cloacal opening; _h_, sub-cuticular nerves running from _c_ to _f_; _k_, spicules.] 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 considerable accumulation of ganglion cells, from which two commissures on each side run into the ventral nerve (Fig. 63, _f_). The ventral branches arise from the ventral nerve-cord immediately in front of the excretory pore. At the posterior end the lateral nerves pass into the two branches into which the ventral nerve divides. Just before the point where the ventral {128}nerve splits it swells out into an anal ganglion situated just in front of the anus. In the male[158] this anal ganglion gives off two lateral nerves which pass round the cloaca and form a ring, and in this sex the ventro-lateral nerve, which is much strengthened by fibres from the ventral nerve, and has received, owing to the mistaken impression that it was a special _nervus recurrens_, the name of the "bursal nerve," gives off numerous branches to the sense papillae which are found in this region of the body and on the tail. The arrangement of these parts is shown in Fig. 63.