258 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



very large axis cylinders, which have been termed neuro- 

 chords. They are not present in Lineus gesserensis. 



In addition to the central nervous system, consisting of 

 the brain and lateral cords, various peripheral nerves may 

 be distinguished. These may be classed under five 

 headings : — 



(1) Cephalic nerves (PI. I., fig. 2, en.), given off 

 anteriorly from the dorsal ganglion and innervating the 

 skin of the snout, the frontal organs, and eyes. 



(2) The oesophageal nerves (PI. II., fig. 1, oesn.) which 

 come off from the hinder portion of the ventral ganglia, 

 and may be regarded as marking the boundary between 

 the latter and tlfe lateral cords. Immediately after 

 coming off the oesophageal nerves of each side unite by 

 several commissures (PI. I., fig. 6, oesc). Behind this 

 the nerves may be easily traced for a little way along 

 the oesophagus, where they lie ventrally and somewhat 

 laterally. Just before the excretory region they become 

 broken up, though it is probable that their fine branches 

 extend backwards and innervate the whole of the alimen- 

 tary canal. By some writers these nerves are spoken of 

 as the vagus nerves. 



(3) The nervous sheath (PI. II., fig 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 dorsal line (PI. III., fig. 8, nd) a thickening 

 of this layer occurs. This is the median dorsal nerve 

 which anteriorly fuses with the dorsal commissure of the 

 brain. From this nervous sheath fine fibrils may be 

 traced to the skin and the muscle layers of the body 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 

 structure from which it receives its name. 



