17 



vol V large axis cylinders, which have been termed neuro- 

 rhords. They are not ])ri'sent in 7J/i<ii.<i f/csscrcnsis. 



In addition to the cential neiNoiis svsteni, consistine' of 

 the brain and lateral cords, vaiiovis peripheral nerves may 

 be distinguished. These may be classed under hve 

 headings : — 



(1) Cephalic nei\es (PI. T., tig. 'J, m.), given ot^' 

 anteriorlv fiom the dorsal yaniilion and iiuuM xatin"' the 

 skin of the snont, the fiontal organs, and eves. 



(2) The (esophageal nerves (PL II., hg. 1, ocsii.) whi(di 

 come oft' fiom the Inndei' poition of the ventral ganglia, 

 and may be regarded as marking the boundary between 

 the latter and the lateral cords. Immediately after 

 coming otf the (esophageal nerves of each side unite by 

 several commissures (PI. I., tig. (i, oesc). Behind this 

 the nerves may be easily traced for a little way along 

 the tesophagus, where they lie ventrally and somewhat 

 laterally. Just before the excretory region they become 

 broken up, though it is probable that their tine branches 

 extend backwards and innervate the whole of the alimen- 

 tary canal. By some writers these nerves are spoken of 

 as the vagus nerves. 



(o) The nervous sheath (PI. II., tig '2, nl .) which forms 

 a delicate coat lying at the same level as the side stems 

 and completely enveloping the circular muscle layer. In 

 the median doisal line (PI. III., fig. 8, n<l) a thickening 

 of this layer occuis. This is the median dorsal nerve 

 which anteriorly fuses with the dorsal commissure of the 

 brain. From this nervous sheath fine tibrils may be 

 traced to the skin and Ihe muscl(> layers of the I)ody wall. 



(4) The proboscis sheath nerve — an exceedingly fine 

 nerve situated just beneath the circular muscle layer in 

 the median dorsal line. It probably innervates the 

 stiucture from wliicii it i-eceiv(>s its name. 



