632 



W. M. SMALLWOOD AND ELIZABETH G. CLARK 



NERVOUS SYSTEM 



The nervous system consists of two main divisions — the 

 central and the visceral or sympathetic part. The central part 

 of the nervous system, enclosed within a semi-transparent mem- 

 branous capsule, forms a nerve collar around the oesophagus, 

 lying just beneath the blood gland. It is made up of three pairs 

 of large ganglia, — two cerebral, two pleural and two pedal, — 

 a small pair of buccal ganglia, and several minor ones. 



jtx.y 



^^n.mti 



Fig. 5 Central nervous system seen from above and posteriorly, co'ms.cb., 

 cerebral commissure ;co'OTs.pZ.+pd., fused pleural and pedal commissures ;(7n.6«c., 

 buccal ganglion i^n. oes., oesophageal ganglion; gn.olf., olfactory ganglion ;(7n. pd., 

 pedal ganglion; gfn.pL, pleural ganglia; rt.miZ., nerves to mantle; n.olj., olfactory 

 nerve; n.pd., pedal nerves; n.vsc, nerve to sympathetic system: oc, eye; xx, xy, 

 nerves to liver and genitalia. 



The cerebral ganglia are the most anterior pair and are united 

 with each other by a short (0.5 mm.) commissure (co'ms.cb.), 

 as seen in figure 5. From the anterior margin of each projects 

 the almost sessile olfactory ganglion (gn.olf.), which gradually 



