LIGIA. 87 



Nervous System. 



The nervous system (PL III., fig. 4) is composed of a 

 series of paired ganglia, the ganglia of each pair being 

 closely apposed ; the ganglia are connected by distinct 

 commissures. 



The supra-oesophageal or cerebral ganglion (cer. g.) 

 extends across the space between the eyes, anterior and 

 dorsal to the gut. The ganglion cells have large deeply- 

 staining nuclei, and the fibres arising from them decussate 

 and connect the ganglia. In the supra-oesophageal 

 ganglia several lobes can be distinguished. On the dorsal 

 side there is a large pair of lobes, from the sides of which 

 the optic stalks arise. Each of these optic stalks consists 

 of a proximal lobe, connected by closely apposed parallel 

 fibres with a distal lobe, from which the optic fibres arise 

 and run direct to the retinulae. On the ventral sides of 

 the superior lobes a small pair of median lobes is 

 situated ; these are anterior to, and connected with, a larger 

 pair of ventral lobes, the olfactory lobes, from which 

 the large antennal nerves (ant. n.) arise. The supra- 

 oesophageal ganglion is connected with the sub- 

 oesophageal ganglion by a pair of peri-oesophageal com- 

 missures. 



The sub-oesophageal ganglionic mass is perforated 

 near the anterior end by a vertical muscle band. The 

 mouth-parts are innervated by two pairs of nerves (m.p.n.), 

 the first of which arises lateral to the perforation, and the 

 second pair posterior to this, and latero-ventral. A pair 

 of nerves (g.n.) arise posterior to these and run ventrally 

 to the stomach. 



The sub-oesophageal ganglion is connected with the 

 ganglia of the first thoracic segment by a pair of cords, 

 from the middle of each of which a bifurcating nerve 

 arises supplying the muscles of the body. 



